Astronomy Picture of the Day Search Results for "horizon"

APOD: 2009 March 11 - Lunar X
Explanation: The striking X near the center of this lunarscape is easily visible in binoculars or a small telescope. Yet, not too many have seen it. The catch is, this lunar X is only apparent during a four hour period just before the Moon's first quarter phase. At the terminator, or shadow line between lunar day and night, the X illusion is produced by a configuration of the craters Blanchinus, La Caille and Purbach. Near the Moon's first quarter phase, an astronaut standing close to the craters' position would see the slowly rising Sun very near the horizon. Temporarily, the crater walls would be in sunlight while the crater floors were still in darkness. Seen from planet Earth, contrasting sections of bright walls against the dark floors by chance look remarkably like an X. This sharp image of the Lunar X was captured at approximately 11:59 UT on March 3, 2009. The Moon's first quarter phase was at 7:46 UT on March 4.

APOD: 2009 March 8 - Gibbous Europa
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA and NASA have together started preliminary development of the Europa Jupiter System Mission , a spacecraft proposed to better study Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

APOD: 2009 February 26 - Moon, Mercury, Jupiter, Mars
Explanation: When the Moon rose in predawn skies on February 23rd, it sported a sunlit crescent. It also offered early morning risers a tantalizing view of earthshine, the dark portion of the lunar disk illuminated by sunlight reflected from the Earth. Of course, on that morning a remarkable conjunction with three wandering planets added an impressive touch to the celestial scene. Recorded just before sunrise, this serene skyview looks east toward a glowing horizon across Tuggerah Lake on the Central Coast of New South Wales, Australia. Along with the waning crescent Moon, the picture captures (top to bottom) bright Mercury, Jupiter, and Mars.

APOD: 2009 February 23 - An Etruscan Vase Moon Rising
Explanation: What's happened to the Moon? Nothing, although from some locations, February's full moon, which occurred about two weeks ago, appeared strangely distorted as it rose. Visible in particular was a curiously inverted image section pinched off near the horizon, an effect dubbed the Etruscan vase by the pioneering science fiction writer Jules Verne for its familiar shape. This odd moon image piece was created by moonlight refracting through an atmospheric inversion layer on Earth where cold air was trapped near the surface. The photographer also reported that, as the moon rose, a red rim was faintly visible on the lower part of the moon, while a green rim appeared on the top. Similar to the Sun's famous green flash, these effects arise when the Earth's atmosphere acts like a prism, sending different colors of light on slightly different paths. The above image mosaic has been horizontally compressed by computer to fit a standard screen.

APOD: 2009 February 19 - Mauna Kea Milky Way Panorama
Explanation: Aloha and welcome to a breathtaking skyscape. The dreamlike panoramic view looks out from the 4,200 meter volcanic summit of Mauna Kea, Hawai'i, across a layer of clouds toward a starry night sky and the rising Milky Way. Anchoring the scene on the far left is the dome of the Canada-France-Hawaii Telescope (CFHT), with north star Polaris shining beyond the dome to the right. Farther right, headed by bright star Deneb, the Northern Cross asterism is embedded along the plane of the Milky Way as it peeks above the horizon. Both Northern Cross and brilliant white Vega hang over a foreground grouping of cinder cones. Near the center are the reddish nebulae, stars and dust clouds of the central Milky Way. Below, illumination from the city lights of Hilo creates an eerie, greenish glow in the clouds. Red supergiant star Antares shines above the Milky Way's central bulge while bright Alpha Centauri lies still farther right, along the dusty galactic plane. Finally, at the far right is the large Gemini North Observatory. The compact group of stars known as the Southern Cross is just left of the telescope dome. Need some help identifying the stars? Just slide your cursor over the picture, or download this smaller, labeled panorama.

APOD: 2009 February 6 - Space Station in the Moon
Explanation: On February 2nd, a first quarter Moon shone in planet Earth's early evening sky. As seen from a location on the US west coast near Mt. Hamilton, California, the International Space Station also arched above the horizon, crossing in front of the Moon's sunlit surface. The space station's transit lasted 0.49 seconds. This sharp exposure, a well-timed telescopic image, recorded the space station during the transit against the background of the Moon's smooth Mare Serenitatis (Sea of Serenity). The orbital outpost was traveling northwest to southeast (from 2 o'clock to 8 o'clock) at a range of 389 kilometers or about 230 miles. Of course, the Moon itself was 1,000 times farther away. In the remarkable photo, the glinting station also offers a hint of the bluish reflection of earthlight.

APOD: 2009 January 5 - Comet and Meteor
Explanation: This meteor streaking toward the horizon through the early morning sky of January 4th is from the annual Quadrantid meteor shower. Aligned with the shower's radiant point high in the north (off the top of the view), the meteor trail passes to the right of bright bluish star Beta Scorpii. Remarkably, near the top of the trail is a small spot, the fuzzy greenish glow of a comet. Discovered in July of 2007, Comet Lulin (C/2007 N3), is too faint now to be easily seen by the unaided eye, but will likely brighten to become visible to skygazers by late February. The well-timed skyscape featuring both comet and meteor is particularly appropriate as cometary bodies are known to be the origins of planet Earth's annual meteor showers.

APOD: 2009 January 2 - Alpine Conjunction
Explanation: Did you see it? The last conjunction of Moon and bright planets in 2008 featured a young crescent Moon and brilliant Venus in the west after sunset on December 31st. Seen here in dark, clear, mountain air from Mönichkirchen, Austria, are the two celestial beacons that dominate planet Earth's night sky. That pair was hard to miss, but skygazers watching lower along the western horizon in early twilight might also have glimpsed a pairing of Jupiter and Mercury as they both wandered closer to the Sun in the sky at year's end. Still, while this single, 5 second long exposure seriously overexposes the Moon's sunlit crescent, it does capture another planet not visible to the unaided eye. The tiny pinprick of light just above the photographer's head in the picture is the distant planet Neptune.

APOD: 2008 December 31 - The Sky in Motion
Explanation: Still need to come up with a good new year's resolution? Consider one appropriate for 2009, the International Year of Astronomy; just look up -- experience, learn, and enjoy the changing sky. This 4-minute, time-lapse video is composed from a series of 7,000 images highlighting much of what you could see. Arcing through the sky in a stately reflection of planet Earth's own rotation are Moon, Sun and stars. But the sequence also features satellites and meteors streaking overhead, clouds moving along the horizon changing in a beautiful iridescence, and beaming crepuscular rays.

APOD: 2008 December 30 - Home from Above
Explanation: There's no place like home. Peering out of the window of the International Space Station (ISS), astronaut Greg Chamitoff takes in the planet on which we were all born. About 350 kilometers up, the ISS is high enough so that the Earth's horizon appears clearly curved. Astronaut Chamitoff's window shows some of Earth's complex clouds, in white, and life giving atmosphere and oceans, in blue. The space station orbits the Earth about once every 90 minutes. It is not difficult for people living below to look back toward the ISS. The ISS can frequently be seen as a bright point of light drifting overhead just after sunset. Telescopes can even resolve the overall structure of the space station. The above image was taken early last month from the ISS's Kibo laboratory.

APOD: 2008 December 24 - Earthrise
Explanation: Forty years ago, in December of 1968, the Apollo 8 crew flew from the Earth to the Moon and back again. Frank Borman, James Lovell, and William Anders were launched atop a Saturn V rocket on December 21, circled the Moon ten times in their command module, and returned to Earth on December 27. The Apollo 8 mission's impressive list of firsts includes: the first humans to journey to the Earth's Moon, the first to fly using the Saturn V rocket, and the first to photograph the Earth from deep space. As the Apollo 8 command module rounded the farside of the Moon, the crew could look toward the lunar horizon and see the Earth appear to rise, due to their spacecraft's orbital motion. Their famous picture of a distant blue Earth above the Moon's limb was a marvelous gift to the world.

APOD: 2008 December 16 - Orion Dawn Over Mount Nemrut
Explanation: What's that in front of Orion? Forty kilometers north of Kahta, Turkey, lies Mount Nemrut, a mountain adorned with the fragments of vast statues built over 2000 years ago. The stone sculptures once stood nearly 10 meters high and depicted lions, eagles, various ancient gods, and King Antiochus I Theos, who ruled Commagene from 86 BC to 38 BC. Ruins of the bodies of several sitting figures are visible on the hill above, illuminated by moonlight. Zeus' head can be found near the above image's center, while the king's head is seen next closest to the horizon. Visible far in the distance in this image, taken three months ago, is the familiar constellation of Orion. The red patch just below Orion's belt is the Orion Nebula, while the bright star to the left of Orion is Sirius. On the far left, a red and brightening horizon announces that the Sun is beginning to rise.

APOD: 2008 December 14 - Zodiacal Light Over New Mexico
Explanation: An unusual triangle of light is visible this time of year just before dawn. Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles. The triangle is clearly visible in the above image taken from New Mexico, USA, in October. A telescope truss tube assembly and observatory domes of New Mexico Skies fill the foreground of the early morning skyscape. Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic. Zodiacal light is so bright this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block out relatively bright reflecting dust. Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset.

APOD: 2008 December 12 - Lick Observatory Moonrise
Explanation: As viewed from a well chosen location at sunset, October's gorgeous Full Moon rose behind Mount Hamilton, east of San Jose, California. Captured in this lovely telescopic view, historic Lick Observatory is perched on the mountain's 4,200 foot summit, observatory and rising Moon momentarily sharing the warm color of filtered sunlight. Of course, tonight those blessed with clear skies can also enjoy a glorious Full Moon. In fact, tonight's Moon reaches its full phase at 1637 UT, within only a few hours of perigee, the closest point in its elliptical orbit. The close approach really will make December's Full Moon the largest Full Moon of 2008, even when it rises high above the horizon.

APOD: 2008 December 5 - Smile in the Sky
Explanation: At sunset, Monday's western sky showed off stunning colors and dramatic clouds reflected in Brisbane Water on the Central Coast of New South Wales, Australia. It also featured the remarkable conjunction of the crescent Moon, Venus, and Jupiter forming a twilight smiley face. While the gathering of the two bright planets and Moon awed skygazers around planet Earth, astronomer Mike Salway reports taking special pains to record this gorgeous view, braving mosquitos and rain squalls along a soggy shore. His southern hemisphere perspective finds brilliant Venus at the highest point in the celestial grouping. For now, a bright pairing of Venus and Jupiter continues to dominate the western horizon after sunset but the Moon has moved on and tonight is near its first quarter phase.

APOD: 2008 December 3 - A Happy Sky Over Los Angeles
Explanation: Sunday, the sky seemed to smile over much of planet Earth. Visible the world over was an unusual superposition of our Moon and the planets Venus and Jupiter. Pictures taken at the right time show a crescent Moon that appears to be a smile when paired with the planetary conjunction of seemingly nearby Jupiter and Venus. Pictured above is the scene as it appeared from Mt. Wilson Observatory overlooking Los Angeles, California, USA after sunset on 2008 November 30. Highest in the sky and farthest in the distance is the planet Jupiter. Significantly closer and visible to Jupiter's lower left is Venus, appearing through Earth's atmospheric clouds as unusually blue. On the far right, above the horizon, is our Moon, in a waxing crescent phase. Thin clouds illuminated by the Moon appear unusually orange. Sprawling across the bottom of the image are the hills of Los Angeles, many covered by a thin haze, while LA skyscrapers are visible on the far left. The conjunction of Venus and Jupiter will continue to be visible toward the west after sunset during much of this month. Hours after the taking of this image, however, the Moon approached the distant duo, briefly eclipsed Venus, and then moved on.

APOD: 2008 November 22 - From Moonrise to Sunset
Explanation: In this panorama of Earth and sky recorded on Thursday, November 13, the Full Moon rises along the eastern horizon at the far left. Of course, the Full Moon rises at sunset and that Thursday's setting Sun was also captured at the far right. In between, 17 digital images are stitched together to follow the horizon to the south in a lovely twilight portrait of the city of Lisbon, Portugal. The serene view takes in part of the longest bridge in Europe, the Vasco da Gama bridge, beneath the rising Moon and ends at the mouth of the Tagus River looking west toward the sunset and the Atlantic Ocean. The photographer's vantage point was Lisbon's 100 foot high Cristo Rei monument on the south bank of the Tagus, at the foot of the port city's other famous bridge, the Ponte 25 de Abril.

APOD: 2008 November 16 - Anticrepuscular Rays Over Colorado
Explanation: What's happening over the horizon? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Pictured above is a particularly striking set of anticrepuscular rays photographed in 2001 from a moving car just outside of Boulder, Colorado, USA.

APOD: 2008 September 26 - Moon Rays over Byurakan Observatory
Explanation: On September 7th, the first quarter Moon and passing clouds contributed to a dramatic night sky over the Byurakan Astrophysical Observatory. This panoramic view begins at the left looking toward the eastern horizon and the rising stars of the constellation Perseus. Sweeping your gaze to the right (south), you'll find the large observatory dome, housing a 2.6 meter diameter telescope, backlit by lights from nearby Yerevan, capital city of Armenia. Fittingly poised above the observatory dome is the bright, giant star Enif in the high-flying constellation Pegasus. Farther to the right, the brightest celestial beacon just above the clouds is our Solar System's ruling gas giant Jupiter. At the far right, the Moon is nearly hidden by an approaching cloudbank, but the clouds themselves actually cast shadows in the bright moonlight, creating the effect of Moon rays across the evening sky.

APOD: 2008 September 22 - Equinox: The Sun from Solstice to Solstice
Explanation: Today is an equinox, a date when day and night are equal. Tomorrow, and every day until the next equinox, the night will be longer than the day in Earth's northern hemisphere, and the day will be longer than the night in Earth's southern hemisphere. An equinox occurs midway between the two solstices, when the days and nights are the least equal. The picture is a composite of hourly images taken of the Sun above Bursa, Turkey on key days from solstice to equinox to solstice. The bottom Sun band was taken during the winter solstice in 2007 December, when the Sun could not rise very high in the sky nor stay above the horizon very long. This lack of Sun caused winter. The top Sun band was taken during the summer solstice in 2008 June, when the Sun rose highest in the sky and stayed above the horizon for more than 12 hours. This abundance of Sun caused summer. The middle band was taken during the Vernal Equinox in 2008 March, but it is the same sun band that Earthlings will see today, the day of the Autumnal Equinox.

APOD: 2008 September 12 - Planets over Perth
Explanation: A bright trio of terrestrial planets was joined by a young Moon on September 1st, in planet Earth's early evening skies. In this view of the celestial gathering from Perth, Western Australia, the Moon's sunlit crescent is nearly horizontal at Perth's southern latitude of about 32 degrees. Venus, then Mercury, and finally Mars shine above colorful city lights on the far shore of the Swan River. The six unlit towers on the left surround a large cricket stadium. For now, the planetary trio still lingers low in the west just after sunset. But in the coming days Venus will move farther from the Sun, climbing higher after sunset, while Mercury and Mars will steadily sink into the glare along the western horizon.

APOD: 2008 September 11 - Mountain Top Meteors
Explanation: A mountain top above the clouds and light-polluted cities was a good place to go to watch this August's Perseid meteor shower. In fact, this composite picture from one of the highest points in Romania, the Omu summit (2,507 meters) in the Southern Carpathian Mountains, captures about 20 of the shower's bright streaks against a starry night sky. The cosmic debris stream that creates the shower is composed of dust particles moving along parallel paths, following the orbit of their parent comet Swift-Tuttle. Looking toward the shower's radiant point in the constellation Perseus, perspective causes the parallel meteor streaks to appear to diverge. But looking directly away from the radiant point, as in this view, perspective actually makes the Perseid meteors seem to be converging toward a point below the horizon.

APOD: 2008 September 5 - Milky Way Road Trip
Explanation: In search of planets and the summer Milky Way, astronomer Tunç Tezel took an evening road trip. Last Saturday, after driving the winding road up Uludag, a mountain near Bursa, Turkey, he was rewarded by this beautiful skyview to the south. Near the center, bright planet Jupiter outshines the city lights below and the stars of the constellation Sagittarius. Above the mountain peaks, an arcing cloud bank seems to lead to the Milky Way's own cloudy apparition plunging into the distant horizon. In Turkish, Uludag means Great Mountain. Uludag was known in ancient times as the Mysian Olympus.

APOD: 2008 August 16 - Perseid over Vancouver
Explanation: Colorful and bright, the city lights of Vancouver, Canada are reflected in the water in this portrait of the world at night. Recorded on August 12 during the Perseid Meteor Shower, the wide-angle view takes in a large swath along the photographer's eastern horizon. The picture is a composite of many consecutive 2 second exposures that, when added together, cover a total time of an hour and 33 minutes. During that time, stars trailed through the night sky above Vancouver, their steady motion along concentric arcs a reflection of planet Earth's rotation. The dotted trails of aircraft also cut across the scene. Of course, two of the frames captured the brief, brilliant flash of a Perseid fireball as it tracked across the top of the field of view. The large gap in the single meteor trail corresponds to the time gap between the consecutive frames.

APOD: 2008 August 14 - Perseid Trail
Explanation: This bright and colorful meteor flashed through Tuesday's early morning skies, part of the annual Perseid Meteor Shower. The lovely image is one of over 350 frames captured on August 12 from the Joshua Tree National Park, in California, USA . Dust from comet Swift-Tuttle is responsible for the Perseids, creating the northern hemisphere's regular summer sky show. The comet dust is vaporized as it enters the atmosphere at upwards of 60 kilometers per second, producing visible trails that begin at altitudes of around 100 kilometers. Of course, the trails point back to a radiant point in the constellation Perseus, giving the meteor shower its name. Recorded after moonset, the starry background features the bright star Vega on the right. Extending below the western horizon is the faint band of the northern Milky Way.

APOD: 2008 August 10 - The Eagle Rises
Explanation: Get out your red/blue glasses and check out this remarkable stereo view from lunar orbit. Created from two photographs (AS11-44-6633, AS11-44-6634) taken by astronaut Michael Collins during the 1969 Apollo 11 mission, the 3D anaglyph features the lunar module ascent stage, dubbed The Eagle, as it rises to meet the command module in lunar orbit. Aboard the ascent stage are Neil Armstrong and Buzz Aldrin, the first to walk on the Moon. The smooth, dark area on the lunar surface is Mare Smythii located just below the equator on the extreme eastern edge of the Moon's near side. Poised beyond the lunar horizon, is our fair planet Earth.

APOD: 2008 July 29 - The Milky Way Over Ontario
Explanation: Sometimes, after your eyes adapt to the dark, a spectacular sky appears. Such was the case earlier this month over Ontario, Canada, when part of a spectacular sky also became visible in a reflection off a lake. To start, the brightest objects visible are bright stars and the planet Jupiter, seen as the brightest spot on the upper left. A distant town appears as a diffuse glow over the horizon. More faint still, the disk of the Milky Way Galaxy becomes apparent as a dramatic diffuse band across the sky that seems to crash into the horizon far in the distance. In the foreground, a picturesque landscape includes trees, a lake, and a stone wall. Finally, on this serene night in July when the lake water was unusually calm, reflections appear. Visible in the lake are not only reflections of several bright stars, but part of the Milky Way band itself. Careful inspection of the image will reveal, however, that bright stars leave small trails in the lake reflections that do not appear in the sky above. The reason for this is because the above image is actually a digital composite of time-consecutive exposures from the same camera. In the first set of exposures, sky images were co-added with slight rotations to keep the stars in one place.

APOD: 2008 July 18 - Jupiter over Ephesus
Explanation: A brilliant Jupiter shares the sky with the Full Moon tonight. Since Jupiter is near opposition, literally opposite the Sun in planet Earth's sky, Jupiter will rise near sunset just like the Full Moon. Of course, opposition is also the point of closest approach, with Jupiter shining at its brightest and offering the best views for skygazers. Recorded late last month, this moving skyscape features Jupiter above the southeastern horizon and the marbled streets of the ancient port city of Ephesus, located in modern day Turkey. At the left is a temple dedicated to the Roman emperor Hadrian. The beautiful night sky also includes the arc of the northern summer Milky Way. Lights on the horizon are from the nearby town of Selçuk. Clicking on the image will download the scene as a panorama.

APOD: 2008 June 28 - Fireball at Ayers Rock
Explanation: A weekend trip for astrophotography in central Australia can result in gorgeous skyscapes. In this example recorded in March of 2006, the center of our Milky Way Galaxy rises over planet Earth's horizon and the large sandstone formation called Uluru, also known as Ayers Rock. After setting up two cameras to automatically image this celestial scene in a series of exposures, one through a wide-angle and the other through a telephoto lens, photographer Joseph Brimacombe briefly turned his back to set up other equipment. To his surprise, the ground around him suddenly lit up with the brilliant flash of a fireball meteor. To his delight, both cameras captured the bright meteor streak. Highlighted in the telephoto view (inset), the fireball trail shines through cloud banks, just left of Ayers Rock.

APOD: 2008 June 7 - June's Young Crescent Moon
Explanation: Serene skyviews were enjoyed across planet Earth earlier this week with a young crescent Moon low in the western sky just after sunset. Recorded on June 4, this colorful example includes a quiet beach in the foreground with the city lights of Lisbon, Portugal, and the Sintra Mountains along the horizon. Posing between cloud banks, the Moon's slender, sunlit arc represents only about 1 percent of the full lunar disc. The rest of the Moon's nearside is faintly visible though, illuminated by Earthshine. A waxing crescent Moon should also create some lovely western skies at dusk this weekend. The bright star in the sky near tonight's (Saturday's) Moon will actually be the planet Mars. On Sunday the Moon will move closer to a pair of celestial beacons, bright star Regulus and Saturn.

APOD: 2008 May 31 - A View to the Sunset
Explanation: Each day on planet Earth can have a dramatic ending as the Sun sets below the colorful western horizon. Often inspiring, or offering a moment for contemplation, a sunset is perhaps the single most photographed celestial event. Did you recognize this as a picture of one? The image actually is a single exposure of the setting Sun recorded near Wasserberg, Germany on May 11. To create the uncommon sunset view the photographer used a digital camera and a zoom lens (a lens with an adjustable focal length). During the 1/6 second long exposure he smoothly changed the focal length while simultaneously rotating the camera, altering the image scale and orientation. The result transforms an objective depiction of nature into an artistic abstraction.

APOD: 2008 May 26 - A New Horizon for Phoenix
Explanation: This flat horizon stretches across the red planet as seen by the Phoenix spacecraft after yesterday's landing on Mars. Touching down shortly after 7:30pm Eastern Time, Phoenix made the first successful soft landing on Mars, using rockets to control its final speed, since the Viking landers in 1976. Launched in August of 2007, Phoenix has now made the northernmost landing and is intended to explore the Martian arctic's potentially ice-rich soil. The lander has returned images and data initially indicating that it is in excellent shape after a nearly flawless descent. News updates will be available throughout the day.

APOD: 2008 May 16 - Circles in the Sky
Explanation: Gazing skyward on a sunny day in May, photographer Jean-Marc Lecleire captured this engaging display of ice halos forming complete circles in the sky. Recorded with a fish-eye lens from a spot near the grand Château de Chambord in France, the picture looks straight up, spanning almost 180 degrees from horizon to horizon. Surrounding the Sun is a halo formed by sunlight refracting through hexagonal-shaped ice crystals in high, thin clouds. The halo is circular and exactly 22 degrees in radius, but it looks squashed because of the distortion of the extremely wide-angle lens. Surrounding the zenith (the point directly above the observer) and always at the same altitude as the Sun is a lovely parhelic circle, caused by sunlight reflecting from ice crystals with nearly vertical faces. On average more common than rainbows, beautiful ice halos can often be seen in planet Earth's sky by those who know how to look for them.

APOD: 2008 May 9 - Moon Meets Mercury
Explanation: On Tuesday, May 6, while standing on planet Earth and sweeping your binoculars along the western horizon just after sunset, you might have encountered this arresting skyscape. The view features a slender crescent Moon and bright planet Mercury separated on the sky by only about 2 degrees. Cradled in the sunlit lunar crescent, the night side of the Moon is faintly illuminated by earthshine -- sunlight reflected from planet Earth. Of course, the clouds in silhouette and fading twilight colors are common elements in pictures of the sky after sunset, but much less often seen is inner planet Mercury, usually hiding close to the Sun in Earth's sky. Still, the coming week will be a good time to spot Mercury near the western horizon about 30 minutes after sunset. As for the Moon, tonight and tomorrow night the crescent Moon will wander close to Mars in the early evening sky.

APOD: 2008 May 7 - The Gegenschein Over Chile
Explanation: Is the night sky darkest in the direction opposite the Sun? No. In fact, a rarely discernable faint glow known as the gegenschein (German for "counter glow") can be seen 180 degrees around from the Sun in an extremely dark sky. The gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter sized splinters from asteroids and orbit in the ecliptic plane of the planets. Pictured above from last October is one of the most spectacular pictures of the gegenschein yet taken. Here a deep exposure of an extremely dark sky over Paranal Observatory in Chile shows the gegenschein so clearly that even a surrounding glow is visible. In the foreground are several of the European Southern Observatory's Very Large Telescopes, while notable background objects include the Andromeda galaxy toward the lower left and the Pleiades star cluster just above the horizon. The gegenschein is distinguished from zodiacal light near the Sun by the high angle of reflection. During the day, a phenomenon similar to the gegenschein called the glory can be seen in reflecting air or clouds opposite the Sun from an airplane.

APOD: 2008 May 3 - Alborz Mountain Milky Way
Explanation: Snow-capped stratovolcano Mt. Damavand climbs to 5,670 meters (18,598 feet) near the left edge in this panoramic view of the world at night. In the sky to the left of Damavand's peak are the stars of the Big Dipper in Ursa Major. Pan to the right and your gaze will sweep across the arch of our Milky Way Galaxy above the Alborz Mountain Range bordering the Caspian Sea. Near the center of the panorama, recorded in the predawn hours of April 4th, bright stars Deneb and Altair lie close to the curve of the Milky Way, above the glow of the Haraz valley. Farther right, brilliant Jupiter dominates the sky near the stars, nebulae, and dark dust clouds toward the bulging galactic center. Finally, the horizon glow at the right edge, below bright yellowish giant star Antares, is from the city of Damavand, named for the legendary mountain peak.

APOD: 2008 April 15 - Sky Delights Over Sweden
Explanation: This night was a sky enthusiast's delight. While relaxing in Sweden last week, many a cosmic wonder was captured with a single snapshot. They are described here from near to far. In the foreground are nearby trees and more distant snow covered mountains. In silhouette, Clouds can be seen just above the horizon, and a careful eye can even discern the more distant green and red auroras which occur in Earth's upper atmosphere. Red emission nebulas dot the sky, including the Heart and Soul Nebulas, IC 1396 and the North America Nebula. Running diagonally from the upper left to the lower right is the majestic glowing band of our Milky Way Galaxy's central plane.. More distant than everything else, appearing as it did over two million years ago, is the Andromeda galaxy, visible above the horizon toward on the lower left.

APOD: 2008 February 28 - ISS: Sunlight to Shadow
Explanation: Orbiting 400,000 kilometers above the Earth, the Moon slid into Earth's shadow to begin last week's total lunar eclipse. Of course the International Space Station (ISS) slides into Earth's shadow every 90 minutes, the time it takes it to complete one orbit at an altitude of about 400 kilometers. Recorded near sunset on February 7, looking toward the north, this composite of 70 exposures shows the trail of the ISS (with gaps between exposures) as it moved left to right over the city of Tübingen in southern Germany. Beginning in sunlight on the left, the ISS vanishes as it enters Earth's shadow at the far right, above the northeastern horizon. As seen from Tübingen, the passage took about 4 minutes. Clicking on the image will download a time-lapse animation (mpg file) based on the individual exposures that includes a plane flying along the horizon.

APOD: 2008 February 16 - Large Binocular Telescope
Explanation: With moonlight on the horizon, a starry sky and the northern Milky Way provide the background for this dramatic view of the World at Night. The imposing structure in the foreground houses the Large Binocular Telescope (LBT), on Mount Graham, Arizona. Inside, the two 8.4 meter diameter mirrors of the LBT really are side-by-side on a common mount, an arrangement mimicking the design of more modest optical equipment usually carried around the neck. While not exactly portable, the benefits of the large scale binocular configuration adopted include an increase in sensitivity over a single mirror telescope and high resolution imaging for faint objects over a relatively wide field of view. An international collaboration operates the LBT Observatory.

APOD: 2008 January 27 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. Mercury is currently visible in the western sky after sunset, but will disappear in the Sun's glare after a few days.

APOD: 2008 January 23 - Orbiting Astronaut Reflects Earth
Explanation: Astronaut self-portraits can be particularly interesting. Visible in the above picture, working in from the outer borders, are the edges of the reflecting helmet of a space suit, modules of the International Space Station (ISS), the Earth, the arms of Expedition 15 astronaut Clay Anderson, and the digital camera used to snap the image. This picture was taken during the shuttle orbiter Endeavour's mission to expand the space station last August. The large curvature of the Earth appearing in the visor reflection is not the true curvature of our spherical Earth, but rather an artifact of the curve of the space helmet. Earth's horizon appears only slightly curved when viewed from the height of the ISS -- approximately 400 kilometers. The next space shuttle mission to the space station is currently expected to take place next month and includes the installation of the scientific Columbus Laboratory.

APOD: 2008 January 21 - Mercury's Horizon from MESSENGER
Explanation: What would it look like to fly past Mercury? Just such an adventure was experienced last week by the MESSENGER spacecraft during its first flyby of the strange moon-like world nearest the Sun. Pictured above is the limb of Mercury seen by MESSENGER upon approach, from about 1 1/2 Earth diameters away. Visible on the hot and barren planet are many craters, many appeared to be more shallow than similarly sized craters on the Moon. The comparatively high gravity of Mercury helps flatten tall structures like high crater walls. MESSENGER was able to take over 1,000 images of Mercury which will be beamed back to Earth for planetary geologists to study. The robotic MESSENGER spacecraft is scheduled to fly past Mercury twice more before firing its thrusters to enter orbit in 2011.

APOD: 2008 January 12 - Mercury Chases the Sunset
Explanation: This colorful view of the western sky at sunset features last Wednesday's slender crescent Moon. Of course, when the Moon is in its crescent phase it can never be far from the Sun in the sky. Also always close to the Sun in Earth's sky is innermost planet Mercury, seen here below and right of center against the bright orange glow along the horizon. Mercury is usually difficult to glimpse because of overwhelming sunlight, but increasingly better views of the small planet after sunset will be possible as it wanders farther east of the Sun in the coming days. On January 14th, NASA's MESSENGER spacecraft will have a good view too, as it makes its first Mercury flyby.

APOD: 2008 January 1 - Rays from an Unexpected Aurora
Explanation: This aurora was a bit of a surprise. For starters, on this day in 2002, no intense auroral activity was expected at all. Possibly more surprising, however, the aurora appeared to show an usual structure of green rays from some locations. In this view, captured from North Dakota, USA, a picket fence of green rays stretches toward the horizon. Mirroring the green rays is a red band, somewhat rare in its own right. Lights from the cities of Bismarck and Mandan are visible near the horizon.

APOD: 2007 December 22 - Tyrrhenian Sea and Solstice Sky
Explanation: Today the Solstice occurs at 0608 Universal Time, the Sun reaching its southernmost declination in planet Earth's sky. Of course, the December Solstice marks the beginning of winter in the northern hemisphere and summer in the south. When viewed from northern latitudes, the Sun will make its lowest arc through the sky along the southern horizon. So in the north, the Solstice day has the shortest length of time between sunrise and sunset and fewest hours of daylight. This striking composite image follows the Sun's path through the December Solstice day of 2005 in a beautiful blue sky, looking down the Tyrrhenian Sea coast from Santa Severa toward Fiumicino, Italy. The view covers about 115 degrees in 43 separate, well-planned exposures from sunrise to sunset.

APOD: 2007 December 21 - Horizon to Horizon
Explanation: Scroll right and journey from horizon to horizon as your gaze sweeps through the zenith in the night sky over Beg-Meil, France. Recorded on December 13th, the entertaining panorama (image key) covers 210 degrees in 21 separate exposures, beginning on the beach with bright star Sirius rising in the southeast. Look up (pan right) to encounter the nebula rich constellation of Orion and continue on to find the lovely Pleiades star cluster. Farther along, higher in the sky, is the famous Comet Holmes, still gracing the northern hemisphere's night with its remarkable expanding coma. Finally, just before diving into the urban glow from city lights along the northwestern horizon (far right), check out the double star cluster in Perseus and take in the cosmic streak of a bright Geminid Meteor.

APOD: 2007 October 11 - Bright Planets, Crescent Moon
Explanation: Early risers are currently enjoying the sight of dazzling Venus, near the eastern horizon as the morning star. Recorded on October 7, this predawn skyview does feature Venus at the upper right. It also includes a crescent Moon and Saturn (lower left). In fact, holding your fist at arms length would have easily covered both planets and the Moon in this 5 degree wide field. Earthshine, sunlight reflected from planet Earth's dayside, illuminates features on the lunar nightside. A close inspection of Saturn itself reveals a nearby pinpoint of light corresponding to Saturn's large moon Titan. Though the Moon has moved on, the tight triangle formed by Venus, Saturn, and Regulus (top), alpha star in the constellation Leo, will continue to look impressive in early morning skies over the next few days. Early bird astrophotographer Jay Ouellet also described Mars as a "brilliant red diode" in his dark country sky east of Quebec City, Canada.

APOD: 2007 September 25 - Zodiacal Light and the False Dawn
Explanation: An unusual triangle of light will be particularly bright near the eastern horizon before sunrise during the next two months for observers in Earth's northern hemisphere. Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles. The triangle is clearly visible toward the left of the frame taken from the Paranal Observatory in Chile in July. Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic. Zodiacal light is so bright this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block out relatively bright reflecting dust. Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset.

APOD: 2007 July 9 - The Most Distant Sun
Explanation: When is the Sun most distant from Earth? It happened again just this past weekend. A common misconception is that the Sun is most distant during the winter, when it's the coldest. In truth, however, the seasonal temperatures are more greatly influenced by the number of daylight hours and how high the Sun rises. For example, during northern winter, the tilt of the Earth causes the Sun to be above the horizon for a shorter time and remain lower in the sky than in northern summer. The picture compares the relative size of the Sun during Earth's closest approach in January (northern winter) on the left, and in July (northern summer) on the right. The angular size of the Sun is noticeably smaller during July, when it is farther away. If the Earth's orbit was perfectly circular, the Sun would always appear to be the same size. These two solar images were taken from Spain during 2006, but the same effect can be seen in any year from any Earth-bound location.

APOD: 2007 July 4 - Red, White, and Blue Sky
Explanation: Contrasting colors in this beautiful sunset sky were captured on June 30 from Clear Creek Canyon Observatory in central Arizona, USA. The twilight scene includes brilliant Venus as the evening star, with a bright Saturn just above it, shining through thin clouds. The two wandering planets were a mere 1 degree apart or so, about twice the width of the full Moon rising above the eastern horizon on the other side of the sky. In fact, such serene skyviews were possible from all over planet Earth as Venus and Saturn approached a conjunction. Regulus, alpha star of the constellation Leo, is above and to the left of the close planetary pairing. At dusk, lights in tonight's sky will also feature Venus and Saturn low in the west and separated by about 2 degrees.

APOD: 2007 June 3 - Shuttle Plume Shadow Points to Moon
Explanation: Why would the shadow of a space shuttle launch plume point toward the Moon? In early 2001 during a launch of Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Next, just at sunset, the shadow is the longest and extends all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon just happened to be.

APOD: 2007 April 4 - New Horizons at Io
Explanation: Spewed from a volcano, a complex plume rises over 300 kilometers above the horizon of Jupiter's moon Io in this image from cameras onboard the New Horizons spacecraft. The volcano, Tvashtar, is marked by the bright glow (about 1 o'clock) at the moon's edge, beyond the terminator or night/day shadow line. The shadow of Io cuts across the plume itself. Also capturing stunning details on the dayside surface, the high resolution image was recorded when the spacecraft was 2.3 million kilometers from Io. Later it was combined with lower resolution color data by astro-imager Sean Walker to produce this sharp portrait of the solar system's most active moon. Outward bound at almost 23 kilometers per second, the New Horizons spacecraft should cross the orbit of Saturn in June next year, and is ultimately destined to encounter Pluto in 2015.

APOD: 2007 March 30 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. Recorded on January 28, the scene also captures the broad tail and bright coma of Comet McNaught, The Great Comet of 2007.

APOD: 2007 March 23 - Touran Sunrise
Explanation: Clouds covered the eastern horizon on Monday, when the Sun rose over the expansive Touran Wildlife Reserve in northeastern Iran. Of course, on that day the Moon rose with the Sun, creating a widely enjoyed partial solar eclipse. Along with a mountainous horizon, the cloud cover lent a dramatic aspect to this eclipse sunrise and made it possible for astronomer Babak Tafreshi to record these telephoto images without using a filter. Advancing north in planet Earth's sky, the Sun itself was also approaching the equinox, the astronomical marker for the first day of northern hemisphere spring and the beginning of Norouz, the Persian New Year.

APOD: 2007 March 22 - Goa Silhouettes
Explanation: On Moonday, March 19, shortly before the equinox, locations in Asia and the Arctic were favoured by the New Moon's shadow during a partial solar eclipse. Although the view from Goa, India found the eclipsed Sun near the horizon, photographer Joerg Schoppmeyer was still able to capture this lovely image, combining celestial with terrestrial silhouettes. The next eclipse season will begin in late August this year, featuring a total lunar eclipse on August 28, and another partial solar eclipse on September 11. Compared to the March 19th eclipse, the September 11th eclipse will be seen on the other side of our fair planet, from parts of South America and Antarctica.

APOD: 2007 March 20 - A Blue Crescent Moon from Space
Explanation: What's happening to the Moon? Drifting around the Earth in 2006 July, astronauts from the International Space Station (ISS) captured a crescent Moon floating far beyond the horizon. The captured above image is interesting because part of the Moon appears blue, and because part of the moon appears missing. Both effects are created by the Earth's atmosphere. Air molecules more efficiently scatter increasingly blue light, making the clear day sky blue for ground observers, and the horizon blue for astronauts. Besides reflecting sunlight, these atmospheric molecules also deflect moonlight, making the lower part of the moon appear to fade away. As one looks higher in the photograph, the increasingly thin atmosphere appears to fade to black.

APOD: 2007 January 24 - A Comet Tail Horizon
Explanation: What's happening over the horizon? Many a sky enthusiast who thought they had seen it all had never seen anything like this. To the surprise of many Northern Hemisphere observers, the tail of Comet McNaught remained visible even after the comet's head set ahead of the Sun. What's more, visible were bright but extremely rare filamentary striae from the comet's expansive dust tail. The cause of dust tail striae are not known for sure, but are possibly related to fragmentation of comet's nucleus. The last comet to show prominent striae was Comet Hale-Bopp in 1997. Pictured above, the tail of Comet McNaught was caught just after sunset last Friday above the Carnic Alps of northern Italy.

APOD: 2007 January 9 - McNaught Now Brightest Comet in Decades
Explanation: The brightest comet in decades is unexpectedly now visible. The most optimistic predictions have Comet McNaught (C/2006 P1) shortly becoming one of the brightest comets of the past century. For the next few days, its short tail and bright coma can be spotted with the unaided eye close to the Sun and near the horizon in both evening and morning skies. This dramatic picture of the comet shining through cloudy skies was taken near sunset on January 7 from Bad Mergentheim, Germany.

APOD: 2007 January 5 - Comet McNaught Heads for the Sun
Explanation: Early morning risers with a clear and unobstructed eastern horizon can enjoy the sight of Comet McNaught (C/2006 P1) in dawn skies over the next few days. Discovered in August by R. H. McNaught (Siding Spring Survey) the comet has grown bright enough to see with the unaided eye but will soon be lost in the glare of the Sun. Still, by January 11 sun-staring spacecraft SOHO should be able to offer web-based views as the comet heads toward a perihelion passage inside the orbit of Mercury. This image captures the new naked-eye comet at about 2nd magnitude in twilight skies near sunset on January 3rd. After rounding the Sun and emerging from the solar glare later this month, Comet McNaught could be even brighter.

APOD: 2006 October 19 - SWAN Meets Galaxy
Explanation: This cosmic portrait recorded October 9th features the lovely blue-green coma of Comet SWAN posing with spiral galaxy NGC 5005 in the northern constellation Canes Venatici. At the time the comet (center) was in the close foreground, a mere 9 light-minutes from planet Earth, with the galaxy a more substantial 60 million light-years distant. Not actually related to a bird, Comet SWAN (C/2006 M4) was so named as it was first spotted in image data from the SWAN (Solar Wind ANisotropies) camera aboard the sun-staring SOHO spacecraft. Having rounded the Sun, this comet is headed for interstellar space, but first it will make its closest approach to Earth on October 24. With binoculars, northern hemisphere observers can now spot the comet above the northwestern horizon, near the handle of the Big Dipper in the early evening sky.

APOD: 2006 October 17 - Clouds and Sand on the Horizon of Mars
Explanation: If you could stand on Mars -- what might you see? Like the robotic Opportunity rover rolling across the red planet, you might well see vast plains of red sand, an orange tinted sky, and wispy light clouds. The Opportunity rover captured just such a vista after arriving at Victoria Crater earlier this month, albeit in a completely different direction from the large crater. Unlike other Martian vistas, few rocks are visible in this exaggerated color image mosaic. The distant red horizon is so flat and featureless that it appears similar to the horizon toward a calm blue ocean on Earth. Clouds on Mars can be composed of either carbon dioxide ice or water ice, and can move quickly, like clouds move on Earth. The red dust in the Martian air can change the sky color above Mars from the blue that occurs above Earth toward the red, with the exact color depending on the density and particle size of the floating dust particles.

APOD: 2006 September 30 - STS 115: Stereo Portrait
Explanation: On September 12, astronaut Heidemarie Stefanyshyn-Piper snapped photos of her colleague Joseph Tanner during the STS-115 mission. At the time, the spacesuited pair were working outside the shuttle orbiter Atlantis, some 300 kilometers above planet Earth. Portions of two of the pictures (S115-E-05750 and S115-E-05753) have been combined in this spectacular 3D image - a stereo anaglyph intended to be viewed with red/blue glasses. Included in the scene reflected in Tanner's visor is Stefanyshyn-Piper herself and the Sun shining above the Earth's distant horizon.

APOD: 2006 September 17 - Anticrepuscular Rays Over Florida
Explanation: What's happening over the horizon? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds. Strangely, the actual sunset was occurring in the opposite direction from where the camera was pointing. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. While enjoying the sunset after visiting NASA's Kennedy Space Center in Florida, the photographer chanced to find that an even more spectacular sight was occurring in the other direction just over the Atlantic Ocean -- a particularly vivid set of anticrepuscular rays.

APOD: 2006 September 6 - Green Aurora Over Lake Superior
Explanation: What if your horizon was green? If you've got a camera, take a picture! That was the experience of Jeff Hapeman last week when visiting the Pictured Rocks National Lakeshore in Michigan. On a quiet night toward the northern horizon over Lake Superior was a long lasting diffuse green aurora. The above image was taken in an effort to capture the sense of wonder one gets when watching an auroral display. Auroras are sparked by energetic particles from the Sun impacting the magnetic environment around the Earth. Resultant energetic particles such as electrons and protons rain down near the Earth's poles and impact the air. The impacted air molecules temporarily lose electrons, and when oxygen molecules among them reacquire these electrons, they emit green light. Auroras are known to have many shapes and colors.

APOD: 2006 July 24 - The International Space Station on the Horizon
Explanation: This was home. Last week, the STS-121 crew of the Space Shuttle Discovery undocked from the International Space Station (ISS) and returned to Earth. As the shuttle departed the space station, they took the above image. Visible on the ISS are numerous modules, trusses, and long wing-like solar panels. The space shuttle crew spent over 12 days calling the space station home. The shuttle crew resupplied the space station and prepared it for future assembly. The ISS's crew of two was expanded to three by the shuttle visit, and now includes one Russian, one American, and one European.

APOD: 2006 July 23 - The Belt of Venus over the Valley of the Moon
Explanation: Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink. Called the Belt of Venus, this off-color band between the dark eclipsed sky and the blue sky can be seen in nearly every direction including that opposite the Sun. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. The Belt of Venus can be seen from any location with a clear horizon. Pictured above, the Belt of Venus was photographed above morning fog in the Valley of the Moon, a famous wine-producing region in northern California, USA. The belt is frequently caught by accident in other photographs.

APOD: 2006 July 20 - Constellation Construction
Explanation: This lovely twilight scene, recorded last April, finds a young crescent Moon low in the west at sunset. Above it, stars shine in the darkening sky but they too are soon to drop below the western horizon. These stars and constellations are prominent in the northern hemisphere winter sky and as the season changes, slowly give way to the stars of summer. Sliding your mouse over the picture will detail the constellations and stars in view, including Orion, Gemini, Auriga, Perseus, and the Hyades and Pleiades star clusters.

APOD: 2006 July 19 - Reflections on Planet Earth
Explanation: Catching sight of your reflection in a store window or shiny hubcap can be entertaining and occasionally even inspire a thoughtful moment. So consider this reflective view from 300 kilometers above planet Earth. The picture is actually a self-portrait taken by astronaut Michael Fossum on July 8 during a space walk or extravehicular activity while the Discovery orbiter was docked with the International Space Station. Turning his camera to snap a picture of his own helmet visor, he also recorded the reflection of his fellow mission specialist, Piers Sellers, near picture center and one of the space station's gold-tinted solar power arrays arcing across the top. Of course, the horizon of our fair planet lies in background.

APOD: 2006 June 29 - Old Moon and Sister Stars
Explanation: An old crescent Moon shares the eastern sky over Menton, France with the sister stars of the Pleiades cluster in this early morning skyscape recorded just last Friday, June 23rd. (Bright Venus was also near the eastern horizon, but is not pictured here.) Astronomical images of the well-known Pleiades often show the cluster's alluring blue reflection nebulae, but they are washed out here by the bright moonlight. Still, while the crescent Moon is overexposed, surface features can be seen on the dim lunar night side illuminated by earthshine - light from sunlit planet Earth. Of course, you can spot a young crescent Moon in the early evening sky tonight. Having left the Pleiades behind, a lovely lunar crescent now appears in the west, lining up with planets Mars, Saturn, and Mercury along the solar system's ecliptic plane.

APOD: 2006 June 15 - Gordel van Venus
Explanation: Scroll right and enjoy this 180 degree panorama across the South African Astronomical Observatory's hilltop Sutherland observing station. Featured are SAAO telescope domes and buildings, along with the dark, wedge-shaped shadow of planet Earth stretching into the distance, bounded above by the delicately colored antitwilight arch. Visible along the antisunward horizon at sunset, (or sunrise) the pinkish antitwilight arch is also known as the Belt of Venus. In order, the significant structures from left to right house; the giant SALT 11-meter instrument, the internet telescope MONET, the 1.9 meter Radcliffe, the 1.0 meter Elizabeth, a 0.75 meter reflector, a 0.5 meter reflector, a garage, YSTAR, BiSON, ACT, IRSF (open), and a storage building. (Note to SAAO fans: in this east-facing view the planet-hunter SuperWASP south is hidden behind the IRSF.)

APOD: 2006 June 11 - Sunspot Loops in Ultraviolet
Explanation: It was a quiet day on the Sun. The above image shows, however, that even during off days the Sun's surface is a busy place. Shown in ultraviolet light, the relatively cool dark regions have temperatures of thousands of degrees Celsius. Large sunspot group AR 9169 is visible as the bright area near the horizon. The bright glowing gas flowing around the sunspots has a temperature of over one million degrees Celsius. The reason for the high temperatures is unknown but thought to be related to the rapidly changing magnetic field loops that channel solar plasma. Sunspot group AR 9169 moved across the Sun during 2000 September and decayed in a few weeks.

APOD: 2006 June 10 - Moon Over Haleakala
Explanation: A waxing crescent Moon shines over the caldera of dormant volcano Haleakala and observatory domes in this dramatic view from above the clouds. Looking west from Maui, Hawaii on May 31st, the scene also records the lights of Honolulu on the horizon. Near the strongly overexposed crescent is bright planet Saturn, but included in the skyscape are planet Mars and the Gemini stars Castor and Pollux. Of course, skywatchers also found star cluster M44, The Beehive Cluster, in this early evening sky, wedged between Saturn and the Moon. In fact, as it closes with Saturn, Mars will pass in front of the Beehive on June 15, so just keep looking west. (Can't find all the players? Click here for help.)

APOD: 2006 April 8 - Vanishing Umbra
Explanation: During the March 29 total solar eclipse, the Moon's dark central shadow or umbra is vanishing beyond the horizon in this dramatic view of the landscape a few kilometers southeast of Incesu, Anatolia, Turkey. The large, snow covered mountain in the distance is 3,250 meter high volcano Hasan Dag. The foreground is growing brighter as eclipse watchers are just beginning to see rays of sunlight peek around the lunar limb, while the mountains on the horizon, left of Hasan Dag, are still completely shadowed by the Moon. For the watchers along this part of the shadow track, the total phase of the eclipse lasted less than 4 minutes as the umbra raced over them at more than 3,000 kilometers per hour.

APOD: 2006 February 4 - Shadow Set
Explanation: A nearly full Moon and planet Earth's shadow set together in this scene captured on January 13th from snowy Mt. Jelm, home of the Wyoming Infrared Observatory. For early morning risers (and late to bed astronomers), shadow set in the western sky is a daily apparition whose subtle beauty is often overlooked in favor of the more colorful eastern horizon. Extending through the dense atmosphere, Earth's setting shadow is seen in this picture as a dark blue band along the distant horizon, bounded above by a pinkish glow or antitwilight arch. Known as the Belt of Venus, the arch's lovely color is due to backscattering of reddened light from the rising Sun. The setting Moon's light is also reddened by the long sight-line through the atmosphere.

APOD: 2006 February 1 - Venus Just After Sunset
Explanation: Is that Venus or an airplane? A common ponderable for sky enthusiasts is deciding if that bright spot near the horizon is the planet Venus. Usually, an airplane will show itself by moving significantly in a few moments. Venus will set only slowly as the Earth turns. Still, the identification would be easier if Venus did not keep shifting its position each night. Pictured above, Venus was captured 38 different nights during 2005 and 2006 over Bursa, Turkey, when Earth's sister planet appeared exclusively in the evening sky. The average spacing of the images was about five days, while the images were always taken with the Sun about 7 degrees below the horizon. Venus' orbit around the Sun will now confine it to Earth's morning sky until October 2006.

APOD: 2006 January 21 - Apollo 12: Self-Portrait
Explanation: In November of 1969, Apollo 12 astronaut-photographer Charles "Pete" Conrad recorded this masterpiece while documenting colleague Alan Bean's lunar soil collection activities on the Oceanus Procellarum. The image is dramatic and stark. The harsh environment of the Moon's Ocean of Storms is echoed in Bean's helmet, a perfectly composed reflection of Conrad and the lunar horizon. Is it art? Works of photojournalists originally intent on recording the human condition on planet Earth, such as Lewis W. Hine's images from New York City in the early 20th century, or Margaret Bourke-White's magazine photography are widely regarded as art. Similarly many documentary astronomy and space images can be appreciated for their artistic and esthetic appeal.

APOD: 2005 December 24 - Earthrise
Explanation: In December of 1968, the Apollo 8 crew flew from the Earth to the Moon and back again. Frank Borman, James Lovell, and William Anders were launched atop a Saturn V rocket on December 21, circled the Moon ten times in their command module, and returned to Earth on December 27. The Apollo 8 mission's impressive list of firsts includes: the first humans to journey to the Earth's Moon, the first manned flight using the Saturn V, and the first to photograph the Earth from deep space. As the Apollo 8 command module rounded the farside of the Moon, the crew could look toward the lunar horizon and see the Earth appear to rise, due to their spacecraft's orbital motion. The famous picture that resulted, of a distant blue Earth above the Moon's limb, was a marvelous gift to the world.

APOD: 2005 December 9 - December Moon Meets Evening Star
Explanation: If you've been outdoors near sunset, then you've probably noticed Venus low in the west as the brilliant evening star. Sometimes mistaken for a tower light near the horizon, Venus is the third brightest celestial beacon, after the Sun and Moon, in planet Earth's sky. That distinction is particularly easy to appreciate in this peaceful scene featuring the crescent Moon, Venus, and sunset colors captured on December 4th near Albany, Missouri, USA. As this season's evening star, Venus will be at its most brilliant tonight, but as December progresses the bright planet will begin to fall out of the western sky. By early next week, December's Moon will have moved on to meet another bright planet overhead -- Mars.

APOD: 2005 November 25 - Moon Over Antarctica
Explanation: Last week, the nearly Full Moon set along the northern horizon - as seen from Davis Station, Antarctica. The squashed orange pumpkin shape just silhouettes the peak of a distant iceberg in this stunning view. The Moon's apparently squashed shape is due to atmospheric bending of light or refraction - an effect which is more severe closer to the horizon. Skimming low along the stark features of the frozen landscape, the Moon's lower edge appears noticeably more distorted than the upper limb. Along with about 70 others present at Davis Station, Dr. Jim Behrens had a chance to enjoy the view while studying the ongoing detachment of a large iceberg known as "Loose Tooth".

APOD: 2005 November 20 - Rays from an Unexpected Aurora
Explanation: This aurora was a bit of a surprise. For starters, on this Friday morning in August 2002, no intense auroral activity was expected at all. Possibly more surprising, however, the aurora appeared to show an usual structure of green rays from some locations. In the above image, captured from North Dakota, USA, a picket fence of green rays stretches toward the horizon. Mirroring the green rays is a red band, somewhat rare in its own right. Lights from the cities of Bismarck and Mandan are visible near the horizon. Large sunspot groups indicate that activity from an active Sun is relatively likely, possibly causing other streams of energetic particles to cascade onto the Earth and so causing more auroras.

APOD: 2005 November 15 - A Taurid Meteor Fireball
Explanation: Have you ever seen a very bright meteor? Unexpected, this year's Taurid meteor shower resulted in numerous reports of very bright fireballs during the nights surrounding Halloween. Pictured above, a fireball that momentarily rivaled the brightness of the full Moon was caught over Cerro Pachon, Chile by a continuous sky monitor on November 1. Several bright Taurid fireballs are identifiable on the sky movie for that night. The above image is a digitally rectangled version of a circular fisheye frame and shows the entire sky, horizon to horizon. The bright meteor was seen swooping between the directions of the Large and Small Magellanic Clouds. The band of the Milky Way Galaxy crosses the horizon behind the dome of the 8-meter Gemini South Telescope. Taurid meteor fireballs are likely pebble sized debris left by Comet Encke. Over the next week the Leonids meteor shower will peak, although they will need to be seen through the glare of a nearly full Moon. [Disclosure: Robert Nemiroff collaborates on both the Astronomy Picture of the Day and the Night Sky Live projects.]

APOD: 2005 October 31 - A Martian Halloween
Explanation: From sunset to sunrise, an unusually bright yellowish orb will hang in the sky this Halloween: Mars. Yesterday, Earth passed Mars as they orbited the Sun, bringing Mars closer than it will be for the next thirteen years. Tonight though, Mars will be nearly as bright as last night, a beacon of extraterrestrial spookiness. Opposite the Sun, Mars will rise just when the Sun sets, set just when the Sun rises, and be visible the entire night. Mars will not always be the brightest object in tonight's sky, though. Brighter than even Mars, almost spooky Venus will light up the western horizon for a brief time just after sunset. Please have a safe and happy All Hallows Eve.

APOD: 2005 September 17 - The Shadow of Phobos
Explanation: Hurtling through space above the Red Planet, potato-shaped Phobos completes an orbit of Mars in less than eight hours. In fact, since its orbital period is shorter than the planet's rotation period, Mars-based observers see Phobos rise in the west and set in the east - traveling from horizon to horizon in about 5 1/2 hours. These three images from the Mars Global Surveyor (MGS) spacecraft record the oval shadow of Phobos racing over western Xanthe Terra on August 26, 1999. The area imaged is about 250 kilometers across and is seen in panels from left to right as red filter, blue filter, and combined color composite views from the MGS wide-angle camera system. The three dark spots most easily seen in the red filter image are likely small fields of dark sand dunes on crater floors. Standing in the shadow of Phobos, you would see the Martian version of a solar eclipse!

APOD: 2005 August 9 - The Belt of Venus over Elwood Beach
Explanation: Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink. Called the Belt of Venus, this off-color band between the dark eclipsed sky and the blue sky can be seen in nearly every direction including that opposite the Sun. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. The Belt of Venus can be seen from any location with a clear horizon. Pictured above, the Belt of Venus was photographed behind Elwood Beach in Melbourne, Australia. The belt is frequently caught by accident in other photographs.

APOD: 2005 August 5 - HD 188753: Triple Sunset
Explanation: Although it looks like fiction, this artist's vision of sunset on an alien world is based on fact -- the recent discovery of a hot, jupiter-sized planet orbiting in triple star system HD 188753. Only 149 light-years away in the constellation Cygnus, HD 188753's massive planet was detected by astronomer Maciej Konacki after analyzing detailed spectroscopic data from the Keck Observatory. The large planet itself is depicted at the upper left in this imagined view from the well-illuminated surface of a hypothetical rocky moon. From this perspective, the closest, hottest and most massive star in the triple system, a star only a little hotter than the Sun, has set below distant peaks. The two other suns nearing the horizon are both cooler and farther from the large planet. While other hot, jupiter-like planets are known to orbit nearby stars, the "crowded" multiple star nature of this system challenges current theories of planet formation.

APOD: 2005 July 11 - Sunrise Over Kilimanjaro
Explanation: Is the Roof of Africa on fire? A group hiking at 6 am near the top of Mt. Kilimanjaro watched the rising sun peak above the clouds and the horizon light up red. Don't worry -- in this case the highest volcano in Africa is not even erupting. The spectacular sunrise colors are caused by light scattering off the atmosphere and small cloud particles. If all of the scattered light that makes the sky blue were added back into the scene, the sunrise would appear Sun-colored and not so red. A similar light scattering effect involving small airborne dust particles causes sunsets on Mars to be red and has been used to determine the sizes of particles in the rings of Saturn. During this trek in 2000 November, a group of about 30 reached the Kilomanjaro summit after a six-day climb.

APOD: 2005 June 30 - Three Planets from Mt Hamilton
Explanation: Venus, Mercury, and Saturn wandered close together in western evening skies last week. On Saturnday, June 25, astronomer R. Jay GaBany recorded this snapshot of their eye-catching planetary conjunction, from historic Lick Observatory on Mt. Hamilton, California, USA. The view looks toward the Pacific shortly after sunset with the lights of San Jose and the southern San Francisco Bay area in the foreground. Of course, Venus is the brightest of the trio. Mercury is nearby on the right and Saturn is below and left, closest to the horizon. Farther to the right of the planetary triangle are Pollux and Castor, twin stars of Gemini, with Regulus, bright star of the constellation Leo, at the very upper left corner of the picture. In the coming days, Venus and Mercury remain close, while Saturn continues to drop below them, toward the horizon.

APOD: 2005 June 24 - Planets in the West
Explanation: This weekend three planets will grace the western sky, forming a lovely trio easily visible shortly after sunset. Saturday evening in particular will find Saturn, Venus, and Mercury all within a 2 degree circle (about the size of your thumb held at arm's length) above the western horizon. Recorded last Sunday, June 19, this image shows the same three planets arrayed along the ecliptic plane above a Colorado Rocky Mountain skyline. Venus is easiest to pick out of the twilight, the brightest celestial beacon below picture center, with Saturn above and to the left of Venus, and Mercury closest to the horizon, right of prominent Pinnacle Peak. By Saturday, the wandering planets will draw even closer together. For help spotting the planets here, put your cursor over the picture.

APOD: 2005 June 23 - Moonrise, Cape Sounion, Greece
Explanation: The Moon was full this month on June 22nd, only a day after the northern hemisphere's summer solstice. Since this solstice marked the northernmost point of the Sun's annual motion through planet Earth's sky, the full Moon rising near the ecliptic plane opposite the Sun was at its farthest south for the year. Only a month earlier, on May 23rd, astronomer Anthony Ayiomamitis recorded this picture of another southerly full Moon rising above Cape Sounion, Greece. The twenty-four hundred year old Temple of Poseidon lies in the foreground, also visible to sailors on the Aegean Sea. In this well-planned, single exposure, a long telephoto lens makes the Moon loom large, but even without optical aid casual skygazers often find the full Moon looking astonishingly large when seen near the horizon. That powerful visual effect is known as the Moon Illusion.

APOD: 2005 May 28 - Himalayan Horizon From Space
Explanation: This stunning aerial view shows the rugged snow covered peaks of a Himalayan mountain range in Nepal. The seventh-highest peak on the planet, Dhaulagiri, is the high point on the horizon at the left while in the foreground lies the southern Tibetan Plateau of China. But, contrary to appearances, this picture wasn't taken from an airliner cruising at 30,000 feet. Instead it was taken with a 35mm camera and telephoto lens by the Expedition 1 crew aboard the International Space Station -- orbiting 200 nautical miles above the Earth. The Himalayan mountains were created by crustal plate tectonics on planet Earth some 70 million years ago, as the Indian plate began a collision with the Eurasian plate. Himalayan uplift still continues today at a rate of a few millimeters per year.

APOD: 2005 May 20 - Aurora Iowa
Explanation: Early last Sunday morning stars were not the only lights in Iowa skies. The northern lights also shone from the heavens, extending across the midwestern USA and other locations not often graced with auroral displays. The wide-ranging auroral activity was triggered as a coronal mass ejection - an energetic cloud of particles blasted outward from the Sun a few days earlier - collided with planet Earth's magnetosphere. Alerted to conditions ripe for aurora, photographer Stan Richard recorded this aparition over Saylorville Lake, near Des Moines. Bright planet Mars in the constellation Aquarius is above the horizon near the center of the eastward-looking view. While the colorful rays seem to end just above the water, they are actually at altitudes of 100 kilometers or more.

APOD: 2005 March 23 - A Dust Devil Swirling on Mars
Explanation: What is that wisp on the horizon? Scientists think that the slight white apparition is actually a Martian dust devil that was caught swirling across Mars. The above image was taken earlier this month by the robotic rover Spirit. The swirling cloud was found by comparing the above image to a previous image of the same area. Fresh dust devil tracks have been seen on Mars before, but actually seeing one up close was a surprise. The most similar phenomena to Martian dust devils on Earth are terrestrial dust devils, tornadoes and waterspouts. The ultimate cause of Martian dust devils remains unknown, but might be related to rising air heated by sun-warmed rocks and soil. Just the previous day, Spirit's power acquisition increased unexpectedly, possibly the result of a dust devil passing near or over the Spirit rover and effectively cleaning its solar panels.

APOD: 2005 March 18 - Moon, Mercury, Monaco
Explanation: Low on the western horizon after sunset, a slender crescent Moon and wandering planet Mercury join the lights of Menton and Monaco along the French Riviera. Astronomer Vincent Jacques took advantage of this gorgeous photo opportunity a week ago on March 11, when the Moon and Mercury were separated in the sky by just three degrees. Of course, the Moon in a slender crescent phase is always seen near the horizon, as is Mercury - a bright planet which can be otherwise difficult to glimpse as it never strays far from the Sun in Earth's sky. In the coming days good views of Mercury will indeed be fleeting as the solar system's innermost planet is rapidly dropping closer to the glare of the setting Sun. But tonight a waxing Moon will join another bright planet wandering overhead through the evening sky, Saturn.

APOD: 2005 March 7 - Mercury Over Leeds
Explanation: Have you ever seen the planet Mercury? This week might be a good time. Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an clear horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2004. Each picture was taken from the same location in Leeds, England exactly 33 minutes after sunset. Over the next two weeks, Mercury will again be well placed for viewing above the western horizon at sunset, but by the third week in March it will have faded and dropped into the twilight.

APOD: 2005 February 17 - Melas, Candor, and Ophir: Valleys of Mariner
Explanation: First imaged by the Mariner 9 spacecraft, Valles Marineris, the grand canyon of Mars, is a system of enormous depressions or chasmas that stretch some 4,000 kilometers along the Martian equator. Looking north over the canyon's central regions, dark Melas Chasma lies in the foreground of this spectacular perspective view. Behind it are Candor Chasma and the steep walls of Ophir Chasma near the horizon. Faulting, surface collapse and landslides are seen to be part of the complex geologic history of these dramatic features, with layered deposits also found within the canyon system. Recorded in 2004, the image represents data from the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft. Melas, Candor and Ophir are about 200 kilometers wide and 5 to 7 kilometers deep.

APOD: 2005 January 29 - Southern Cross in Mauna Loa Skies
Explanation: Gazing across this gorgeous skyscape, the Southern Cross and stars of the constellation Centaurus are seen above the outline of Mauna Loa (Long Mountain), planet Earth's largest volcano. Unfamiliar to sky gazers north of about 25 degrees north latitude, the Southern Cross, constellation Crux, is near the horizon to the left of Mauna Loa's summit. A compact constellation of bright stars, the long axis of the cross conveniently points south toward the southern celestial pole. The top of the cross is marked by the lovely pale red star Gamma Crucis, which is in fact a red giant star about 120 light-years distant. Stars of the grand constellation Centaurus almost engulf the Southern Cross with blue giant Beta Centauri, and yellowish Alpha Centauri, appearing as the brightest stars to the left of Gamma Crucis. At a distance of 4.3 light-years, Alpha Centauri, the closest star to the Sun, is actually a triple star system which includes a star similar to the Sun. But what caused the reddish streaks in the foreground of this time exposure? Alas, it is the mundane glow of lights from cars (not molten lava!) traveling the road to Hilo, Hawaii.

APOD: 2005 January 27 - Shadow Set
Explanation: A nearly full Moon and planet Earth's shadow set together in this scene captured Monday from snowy Mt. Jelm, home of the Wyoming Infrared Observatory. For early morning risers (and late to bed astronomers), shadow set in the western sky is a daily apparition whose subtle beauty is often overlooked in favor of the more colorful eastern horizon. Extending through the dense atmosphere, Earth's setting shadow is seen in this picture as a dark blue band along the distant horizon, bounded above by a pinkish glow or antitwilight arch. Also known as the Belt of Venus, the arch's lovely color is due to backscattering of reddened light from the rising Sun. The setting Moon's light is also reddened by the long sight-line through the atmosphere and echoes the dawn sky's yellow-orange hues.

APOD: 2004 December 9 - Jupiter and the Moon's Shadowed Horizon
Explanation: Early Tuesday morning, December 7th, June Croft thought the southeastern sky above Atmore, Alabama, USA was beautiful. Watching the Moon rise through gossamer clouds, she noted, " ... the crescent Moon looked like it was held in the sky by a star just off its shadowed horizon." What was that star? Bright Jupiter of course, and some watched as the Moon actually occulted or passed in front of the Solar System's reigning gas giant planet. For astronomer Jimmy Westlake in Colorado, Jupiter was already hidden at moonrise that morning, but later he was able to record this lovely image, not unlike the view that inspired Croft. Seen through gossamer clouds, Jupiter along with large Jovian satellites Ganymede and Callisto (bottom to top) has emerged from behind the crescent Moon's shadowed horizon.

APOD: 2004 November 13 - Moon Over Shiraz
Explanation: Early morning risers around the world have enjoyed the sight of bright planets in this week's predawn skies - further enhanced by the celestial spectacle of the waning crescent Moon. From some locations the Moon was seen to pass in front of Jupiter or Venus, a lunar occultation. Recorded near sunrise on November 10th from Shiraz, Iran, this eastern horizon view finds Jupiter (top) and a brilliant Venus in line with the Moon, a lovely conjunction of the three brightest objects in the night sky. Although the Moon has now fallen out of the early morning scene, Venus and Jupiter (along with a much fainter Mars) still precede the rising Sun above the eastern horizon.

APOD: 2004 November 12 - Missouri's Green Ribbon Sky
Explanation: The critics rave - "Amazing!", "Unbelievable!", "The best I've ever seen!" They aren't talking about a movie, though. Instead, even casual sky critics are remarking on November's stunning auroral displays, visible with surprising intensities well beyond the confines of high latitudes where auroral activity is normally observed. In fact, in this example of an unforgettable performance a green ribbon of auroral light stretches from horizon to horizon - recorded on November 7th with a fisheye lens near Warrensburg, Missouri, USA. Want to see an aurora? Relatively wide-spread displays may continue, triggered by activity from an energetic sunspot region.

APOD: 2004 October 10 - Sunspot Loops in Ultraviolet
Explanation: It was a quiet day on the Sun. The above image shows, however, that even during off days the Sun's surface is a busy place. Shown in ultraviolet light, the relatively cool dark regions have temperatures of thousands of degrees Celsius. Large sunspot group AR 9169 is visible as the bright area near the horizon. The bright glowing gas flowing around the sunspots has a temperature of over one million degrees Celsius. The reason for the high temperatures is unknown but thought to be related to the rapidly changing magnetic field loops that channel solar plasma. Sunspot group AR 9169 moved across the Sun during 2000 September and decayed in a few weeks.

APOD: 2004 September 11 - The Star Trails of Kilimanjaro
Explanation: The night had no moon, but the stars were out. And camped at 16,000 feet on Mt. Kilimanjaro, photographer Dan Heller recorded this marvelous 3 1/2 hour long exposure. Here the landscape is lit mostly by the stars. Flashlights give the tents an erie internal radiance while the greenish glow from the distant city lights of Moshi, Tanzania filter through the clouds below. The view from this famous equatorial African mountain is toward the south, putting the South Celestial Pole close to the horizon on the far left, near the center of the graceful concentric star trail arcs. In the thin air and clear dark skies, even the ghostly Milky Way left a faint triangular glow as it swept across the middle of the dreamlike scene.

APOD: 2004 August 25 - Zodiacal Light and the False Dawn
Explanation: An unusual triangle of light will be particularly bright near the eastern horizon before sunrise during the next two months for observers in Earth's northern hemisphere. Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles. The triangle is clearly visible in the left frame taken in Namibia in May. Rolling the cursor over the image will bring up labels. Bright zodiacal light can be seen nearly every clear moonless morning over the next few months on images taken by the Mauna Kea, Hawaii fisheye CONCAM of the Night Sky Live project. Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic. Zodiacal light is so bright this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block out relatively bright reflecting dust. Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset.

APOD: 2004 June 6 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk on 2003 May 7, as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century, but the next similar event will be a much more rare transit of Venus this coming Tuesday. Need help spotting Mercury? Just click on the picture.

APOD: 2004 June 4 - Sedna at Noon
Explanation: Standing on Sedna - the solar system's most distant known planetoid - your view of the Sun at high noon might look something like this. An artist's dramatic vision, the picture shows the Sun suspended above the nearby horizon as a bright star immersed in the dusty ecliptic plane. Within the dust-scattered sunlight are more familiar members of the solar system, including planet Earth. But at a distance of about 13 billion kilometers (8 billion miles) Earth would only be visible in binoculars or a small telescope. In Sedna's dark, daytime skies, the noonday Sun is also joined by the faint stars and obscuring dust clouds of the Milky Way, suspended on the left above stark, ruddy terrain. For Sedna-based sky gazers, all planets have interior orbits and would remain close to the Sun in Sedna's skies. Of course, for earthbound astronomers, interior planets Venus and Mercury also remain near the Sun, with Venus scheduled for a rare crossing of the solar disc on June 8.

APOD: 2004 May 27 - Two Comets in Southern Skies
Explanation: Wielding a very wide-angle lens, astronomer Gordon Garradd was able to capture two naked-eye comets in one picture looking toward the west from Loomberah, New South Wales, Australia. At the far left lies comet C/2002 T7 (LINEAR) and at the far right, comet C/2001 Q4 (NEAT). Recorded on the night of May 20th, the area around each of the comets has been separately enhanced here, making it easier to discern their extended tails streaming away from the Sun. While comet T7 (LINEAR) is the brighter of the two comets, both are now fading. Still, they may be visible for northern and southern observers with Q4 (NEAT) easiest to spot in the north. Of course, with bright Venus near the center on the horizon and the lights of nearby Tamworth city glowing at the bottom right, the two comets are not alone in this heavenly view.

APOD: 2004 May 25 - Moon Between the Stones
Explanation: Despite clouds and rain showers astronomer Phillip Perkins managed to spot a reddened, eclipsed Moon between the stones of this well known monument to the Sun during May's total lunar eclipse, from Stonehenge, England. When he recorded this dramatic picture, the rising Moon was only about 5 degrees above the horizon, but conveniently located through a gap in the circle of ancient stones. Although at first glance there appears to be an eerie, luminous pool of water in the foreground, Perkins notes that his daughter produced the artistic lighting effect. She illuminated a fallen stone and surrounding grass with a flashgun from her hiding place behind the large sarsen stone to the right of center. As the picture looks toward the southeast, the stone just below the Moon is one of the inner bluestones rather than the famous Heel Stone, which marks the northeast direction of the summer solstice sunrise.

APOD: 2004 May 1 - A Western Sky at Twilight
Explanation: On April 23rd, the Moon along with planets Saturn, Mars, and Venus (and planet Earth of course ...) were all visible in the west at twilight, captured here from a site near Saylorvillle Lake north of Des Moines, Iowa, USA. Putting your cursor over the image will label our fellow solar system wanderers and also reveal the approximate trajectory of the ecliptic plane - defined by Earth's orbit around the Sun - angling above the western horizon. After sunset tonight, the western sky will present a similar arrangement of planets, although the Moon will have moved east out of the picture, passing bright Jupiter along the ecliptic and heading for May 4th's total lunar eclipse. May could also be a good month for comets.

APOD: 2004 April 1 - April Fools Day More Intense On Mars
Explanation: Today, April 1st, astrophysicists have announced a surprising discovery - April Fools Day is more intense on Mars! Though the discovery is contrary to accepted theories of April Fools Day, researchers note that there are several likely causes for the severe martian April Fools phenomenon. For starters, gravity, the force that opposes comedy throughout the universe, is only about 3/8ths as strong on Mars' surface as it is on planet Earth. Also, a martian day, called a sol, lasts nearly 40 minutes longer than an earth day. And furthermore ... well, as soon as they think of some more reasons, they've promised to tell us. Happy April Fools day from the editors at APOD! Editors note: Mars rover Spirit recorded this image looking out toward the eastern horizon and the Columbia Hills over 2 kilometers in the distance. Its journey across this rocky martian terrain could take from 60 to 90 sols.

APOD: 2004 March 26 - Moon and Planets Sky
Explanation: Look up into the sky tonight and without a telescope or binoculars you might have a view like this one of Moon, planets and stars. The lovely photo was taken on March 23rd, and captures the crescent Moon on the horizon with Venus above it. Both brilliant celestial bodies are over-exposed. Farther above Venus is the tinted glow of Mars with the Pleiades star cluster just to the red planet's right. The V-shaped arrangement of stars to the left of Mars is the Hydaes star cluster. Bright red giant Aldebaran, not itself a member of the Hyades cluster, marks the top left of the V. During the next week, all five naked-eye planets, Mercury, Venus, Mars, Jupiter, and Saturn, along with the Moon will grace the evening sky together - a lunar and planetary spectacle that can be enjoyed by skygazers around the world. But look just after sunset, low on the western horizon, to see Mercury before it sets. The next similar gathering of the planets will be in 2008.

APOD: 2004 March 24 - Intriguing Dimples Near Eagle Crater on Mars
Explanation: What are those unusual looking dimples? Looking back toward Eagle crater, its landing place on Mars, the robot rover Opportunity has spotted some unusual depressions in the Martian soil. The dimples, visible above on the image left, each measure about one meter across and appear to have light colored rock in their interior. The nearest dimple has been dubbed "Homeplate", while the next furthest one out is called "First Base." Scrolling right will reveal a magnificent panorama including the rover in the foreground, the backshell and parachute that detached from Opportunity before it landed near the horizon, Eagle crater in the center, Opportunity's tracks as it rolled away from Eagle crater, and wind blown ripples of Martian soil in every direction. Further analysis of rocks photographed by Opportunity has yielded evidence that Opportunity has landed on an evaporated shoreline of an ancient salt-water ocean.

APOD: 2004 March 20 - Equinox + 1
Explanation: Twice a year, at the Spring and Fall equinox, the Sun rises due east. In an emphatic demonstration of this celestial alignment, photographer Joe Orman recorded this inspiring image of the Sun rising exactly along the east-west oriented Western Canal, in Tempe,Arizona, USA. But he waited until March 21st, one day after the northern Spring equinox in 2001, to photograph the striking view. Why was the rising Sun due east one day after the equinox? At Tempe's latitude the Sun rises at an angle, arcing southward as it climbs above the horizon. Because the distant mountains hide the true horizon, the Sun shifts slightly southward by the time it clears the mountain tops. Waiting 24 hours allowed the Sun to rise just north of east and arc back to an exactly eastern alignment for the photo. Today's equinox finds the Sun on the celestial equator at 0649 Universal Time.

APOD: 2004 February 4 - Opportunity's Horizon
Explanation: Remarkably, the Opportunity Mars rover lies in a small martian impact crater about 3 meters deep and 22 meters wide. For 360 degrees, Opportunity's horizon stretches to the right in this new color mosaic image from the rover's panoramic camera. Notable in this view of the generally dark, smooth terrain are surface imprints left by the lander's airbags and an outcropping of light-colored, layered rock about 8 meters away toward the northwest. Though they look imposing, the rocks in the tantalizing outcrop are only a few centimeters high and will be dwarfed by the cart-sized rover itself during future close-up investigations. Opportunity has now rolled off its lander and, along with the restored Spirit rover, is directly exploring the martian surface.

APOD: 2004 January 5 - Spirit Pan from Gusev Crater
Explanation: After a seven month voyage of nearly 500 million kilometers through interplanetary space, NASA's Spirit Rover has reached the surface of Mars. Scroll right and see a mosaic panorama of Spirit's first images returned from its landing site in Gusev Crater! Taken by Spirit's navigation camera, the panorama covers 360 degrees, with the spacecraft in the foreground and the floor of Gusev Crater, thought to be an ancient lake bed, extending to the horizon. The entry, descent, and landing phase of Spirit's mission - referred to by mission planners as "Six Minutes of Terror" - began Saturday night around 8:30pm PST as Spirit entered the martian atmosphere at about 20,000 kilometers per hour. Updates on Spirit's status will be posted throughout the day.

APOD: 2003 November 25 - A Late Leonid from a Sparse Shower
Explanation: The 2003 Leonids Meteor Shower contained relatively few meteors. As expected and unlike the last few years, the Earth just did not pass through any dense particle streams left over by the Sun-orbiting Comet Tempel-Tuttle. Preliminary reports had the peak meteor rates only as high as about one relatively faint meteor a minute even from good locations at good times. Pictured above is one of the brighter Leonids of 2003, caught by one of the continuously operating night sky web cameras (CONCAMs) of the global Night Sky Live project. The fisheye image shows the night sky from horizon to horizon above Mauna Kea, Hawaii, USA. The image is annotated with several bright stars and planets. Note that this meteor, as do all Leonids, appears to emanate from the constellation Leo, labeled on the upper left. Although the peak of the Leonids this year was on November 19, this meteor flashed through the sky the next night.

APOD: 2003 October 21 - The Belt of Venus over the Valley of the Moon
Explanation: Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink. Called the Belt of Venus, this off-color band between the dark eclipsed sky and the blue sky can be seen in nearly every direction including that opposite the Sun. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. The Belt of Venus can be seen from any location with a clear horizon. Pictured above, the Belt of Venus was photographed above morning fog in the Valley of the Moon, a famous wine-producing region in northern California, USA. The belt is frequently caught by accident in other photographs.

APOD: 2003 October 11 - Moonrise Over Seattle
Explanation: Is the Moon larger when near the horizon? No -- as shown above, the Moon appears to be very nearly the same size no matter its location on the sky. Oddly, the cause or causes for the common Moon Illusion are still being debated. Two leading explanations both hinge on the illusion that foreground objects make a horizon Moon seem farther in the distance. The historically most popular explanation then holds that the mind interprets more distant objects as wider, while a more recent explanation adds that the distance illusion may actually make the eye focus differently. Either way, the angular diameter of the Moon is always about 0.5 degrees. In the above time-lapse sequence taken near the end of 2001, the Moon was briefly re-imaged every 2.5 minutes, with the last exposure of longer duration to bring up a magnificent panorama of the city of Seattle.

APOD: 2003 August 22 - Shadow Rise
Explanation: As the Sun sets, the Earth's shadow rises up from the east. The subtle beauty of this daily apparition is often overlooked in favor of the brighter, more colorful western horizon. But while gazing toward a nearly full rising Moon on August 9, astronomer Steve Mandel admired the shadow rise from his driveway near Soquel, California, USA. His view looks east from the northern tip of Monterey Bay toward Fremont Peak, the highest point in the small mountain range on the horizon. The Earth's rising shadow is cast through the dense atmosphere and is seen in his picture as the dark blue band along the horizon, bounded above by a pinkish purple glow or antitwilight arch. Also known as the Belt of Venus, the arch's lovely color is due to backscattering of reddened light from the setting Sun.

APOD: 2003 May 8 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk yesterday (Wednesday, May 7th), as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This is the first of 14 transits of Mercury which will occur during the 21st century, but the next similar event will be a transit of Venus in June of 2004. Need help spotting Mercury? Just click on the picture.

APOD: 2003 April 12 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. By the middle of this month, Mercury will again be well placed for viewing above the western horizon at sunset, but by the end of April it will have faded and dropped into the twilight. On May 7th, Mercury will cross the Sun's disk.

APOD: 2003 April 6 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1978 Viking 1 orbiter image, the largest moon, Phobos, is indeed seen to be a heavily cratered asteroid-like object. About 17 miles across, Phobos really zips through the Martian sky. Actually rising above Mars' western horizon and setting in the east, it completes an orbit in less than 8 hours. But Phobos orbits so close to Mars, (about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: 2003 March 29 - The Shadow of Phobos
Explanation: Hurtling through space above the Red Planet, potato-shaped Phobos completes an orbit of Mars in less than eight hours. In fact, since its orbital period is shorter than the planet's rotation period, Mars-based observers see Phobos rise in the west and set in the east - traveling from horizon to horizon in about 5 1/2 hours. These three images from the Mars Global Surveyor (MGS) spacecraft record the oval shadow of Phobos racing over western Xanthe Terra on August 26, 1999. The area imaged is about 250 kilometers across and is seen in panels from left to right as red filter, blue filter, and combined color composite views from the MGS wide-angle camera system. The three dark spots most easily seen in the red filter image are likely small fields of dark sand dunes on crater floors. Standing in the shadow of Phobos, you would see the Martian version of a solar eclipse!

APOD: 2003 March 15 - Apollo 12: Self-Portrait
Explanation: Is it art? In November of 1969, Apollo 12 astronaut-photographer Charles "Pete" Conrad recorded this masterpiece while documenting colleague Alan Bean's lunar soil collection activities on the Oceanus Procellarum. The image is dramatic and stark. Bean is faceless. The harsh environment of the Moon's Ocean of Storms is echoed in his helmet's perfectly composed reflection of Conrad and the lunar horizon. Works of photojournalists originally intent on recording the human condition on planet Earth, such as Lewis W. Hine's images from New York City in the early 20th century, or Margaret Bourke-White's magazine photography are widely regarded as art. Similarly many documentary astronomy and space images can be appreciated for their artistic and esthetic appeal.

APOD: 2003 March 6 - Comet NEAT in Southern Skies
Explanation: After last month's dramatic swoop past the Sun, Comet NEAT (C/2002 V1) appeared as a naked-eye comet, emerging from the evening twilight in planet Earth's southern skies. On March 1st, New Zealand photographer Noel Munford captured this telephoto view of the outbound comet close to the southwestern horizon against the faint stars of the constellation Sculptor. He reports that the picture is a good representation of the comet's visual appearance on that date and estimates the impressive tail to be five or six degrees long. Discovered last November as part of the Near-Earth Asteroid Tracking program, there was some speculation that this comet would not survive its close encounter with the Sun. However, Comet NEAT is now returning to the outer solar system, diving southward and fading fast.

APOD: 2003 February 26 - Anticrepuscular Rays Over Horseshoe Canyon
Explanation: What's happening over the horizon? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. While enjoying the sunset after dinner near Horseshoe Canyon in Utah, the photographer chanced to find that an even more spectacular sight was occurring in the other direction just over the canyon -- a particularly vivid set of anticrepuscular rays.

APOD: 2003 February 16 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise.

APOD: 2003 February 10 - Comet NEAT Approaches the Sun
Explanation: A comet may likely become visible to the unaided eye over the next few days above the horizon where the Sun has just set. Comet NEAT (C/ 2002 V1), discovered last November, has brightened dramatically as it approached the Sun. Over the next few days, the quickly setting comet could appear as bright as second magnitude. On February 18 it will round the Sun well within the orbit of Mercury. During surrounding days, the Sun's glare will effectively hide the comet to human observers. It is quite probable, though, that Comet NEAT will standout prominently in images taken by the Sun-looking SOHO satellite. Pictured above, Comet NEAT's complex and developing tail was photographed on January 29 (top) and February 2. Sky enthusiasts should remember to never look directly at the Sun.

APOD: 2003 January 23 - Launch of the Sun Pillar
Explanation: On January 16, NASA's space shuttle Columbia roared into blue morning skies above Kennedy Space Center on STS-107, the first shuttle mission of 2003. But this is not a picture of that launch! It was taken on the morning of January 16 though, at sunrise, looking eastward toward Lake Ontario from just outside of Caledon, Ontario, Canada. In the picture a sun pillar, sunlight reflecting from ice crystals gently falling through the cold air, seems to shoot above the fiery Sun still low on the horizon. By chance, fog and clouds forming over the relatively warm lake look like billowing smoke from a rocket's exhaust plume and complete the launch illusion. Amateur photographer Lauri Kangas stopped on his way to work to record the eye-catching sun pillar launch.

APOD: 2003 January 6 - Shadow Cone of a Total Solar Eclipse
Explanation: Sometimes, during a total eclipse of the Sun, a strange shadow of darkness can be seen stretching off into the distance. Called a shadow cone, they are visible because the Earth's atmosphere is not completely transparent, scattering sunlight and hence appearing blue during the day. Shadow cones are particularly dramatic for eclipses near the horizon, as geometry creates a long corridor of sun-blocked air. Visible above is a shadow cone caught during a total solar eclipse visible last month from South Australia. The eclipsed Sun itself still appears bright because of light from the surrounding corona. The digital camera on the left is zoomed in to show a better image of the actual eclipse.

APOD: 2002 November 15 - Night Trails of Africa
Explanation: Spanning southern to northern skies, stars trail across this panoramic view of the African night from equatorial Kenya. The three hour long exposure was made on a clear, dark, mid November evening facing due west and covers just over 180 degrees along the horizon. So, the South Celestial Pole is at the center of the concentric arcs on the left and the North Celestial Pole is at the far right (scroll right). And, you guessed it(!), the stars setting along the Celestial Equator leave the straight trails near the middle of the picture. Well illustrated in this thoughtfully composed panorama, the star trails in the African night are, of course, not due to motions of the individual stars but simply reflect the daily rotation of planet Earth itself.

APOD: 2002 November 7 - 2001 Leonids: Meteors in Perspective
Explanation: The 2001 Leonid storm was so intense that the meteor shower's radiant, the point on the sky from which the fleeting trails seemed to diverge, was easy to spot. But the bits of debris that created the meteors really moved along parallel paths, following the orbit of their parent comet Tempel-Tuttle. Their apparent divergence from the shower's radiant point was simply due to perspective as skygazers looked toward the stream of cosmic debris. During the 2001 Leonid storm, while the radiant was above the horizon from SoBaekSan Observatory in South Korea, astronomer Christophe Marlot made this single time exposure recording star trail arcs and a number of meteors. Since Marlot was looking away from the cosmic debris stream, this perspective actually shows red tinged meteor trails converging toward a point below the horizon and opposite the radiant -- the Leonid shower's antiradiant.

APOD: 2002 September 15 - Zodiacal Light and the False Dawn
Explanation: An unusual triangle of light will be particularly bright near the eastern horizon before sunrise during the next two months for observers in Earth's northern hemisphere. Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles. The triangle is clearly visible on the left of the above frame taken from Mauna Kea in Hawaii on September 8 by one of the developing global network of fisheye nighttime web cameras, called CONCAMs, of the Night Sky Live Project. Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic. Zodiacal light is so bright this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block out relatively bright reflecting dust. Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset.

APOD: 2002 September 10 - Venus Beyond the Storm
Explanation: A thunderstorm, lightning, a bright star and a bright planet all graced an evening sky for a short while near Bismarck, North Dakota, USA two weeks ago. Thick thunderclouds from a passing storm are the origin of a strong cloud to ground lightning strike. Small areas of rain darken portions of the orange sunset, visible at the horizon above the vast prairie. The planet Venus peeks below the clouds on the lower left of the image. Blue sky shines high above the distant storm, streaked with high white cirrus clouds. The bright star Arcturus glitters near the image top, just left of center. Just a few minutes later, only a memory and this picture remained.

APOD: 2002 August 5 - Rays from an Unexpected Aurora
Explanation: This aurora was a bit of a surprise. For starters, last Friday morning, no intense auroral activity was expected at all. Possibly more surprising, however, the aurora appeared to show an usual structure of green rays from some locations. In the above image, captured from North Dakota, USA, a picket fence of green rays stretches toward the horizon. Mirroring the green rays is a red band, somewhat rare in its own right. Lights from the cities of Bismarck and Mandan are visible near the horizon. Large sunspot groups indicate that activity from the currently active Sun is relatively likely, possibly causing other streams of energetic particles to cascade onto the Earth and so causing more auroras.

APOD: 2002 July 29 - A Setting Sun Trail
Explanation: The Sun appears to move on the sky because the Earth rotates. The extreme brightness of the Sun, however, makes it difficult to capture a sun-trail -- the path the Sun traces on the sky. To capture the above picture, a very dark filter covered the camera lens for most of the time, allowing only a trifle of light from the bright Sun to peek through. Just after the Sun had dipped below the horizon but before it was completely dark, the thick filter was removed and the pretty foreground scene was captured. Slight flares appeared when the Sun went behind thin clouds. Star-trails and planet-trails are much easier to image, and a similar Moon trail has also recently been imaged.

APOD: 2002 June 25 - Venus and Jupiter Over Belfast
Explanation: Venus and Jupiter appeared to glide right past each other earlier this month. In a slow day-by-day march, Jupiter sank into the sunset horizon while Venus remained high and bright. The conjunction ended the five-planet party visible over the last two months. Jupiter, of course, is much further away from the Earth and Sun than Venus, so the passing was really just an angular illusion. Pictured above on June 3, a fading sunset finds Venus shining over Jupiter above clouds, mountains, and the city lights of Belfast, Northern Ireland.

APOD: 2002 June 21 - Zimbabwe Sunset
Explanation: Today's scheduled geocentric astronomical event is the Solstice, with the Sun reaching its northernmost declination at 13 hours 24 minutes Universal Time. For denizens of planet Earth this Solstice marks the beginning of Summer in the northern hemisphere and Winter in the south. Of course, the tilt of the Earth's axis of rotation (and not a change in the Earth-Sun distance) is mainly responsible for the changing seasons and the Sun's yearly north-south motion through the sky. Following the rising and setting points of the Sun along the horizon is one way to track the Sun's progress along its seasonal cycle. Tall grasses and tinted clouds frame this dramatic view of the setting Sun approaching the northern limit of this year's seasonal journey as seen near Raffingora, Zimbabwe.

APOD: 2002 May 16 - Double Trouble Solar Bubbles
Explanation: During April and May, attention has been focused on the western evening sky, presenting its spectacle of bright planets and crescent moons shortly after sunset. Meanwhile, the Sun itself has not been just sinking quietly below the horizon. For example on May 2nd, two enormous clouds of energetic particles blasted away from the solar surface in nearly simultaneous eruptions. Known as coronal mass ejections (CMEs), they appear as large "bubbles" oriented at about 2 o'clock and 8 o'clock in this composite image from cameras onboard the sun-staring SOHO spacecraft. At picture center, an extreme ultraviolet image of the Sun recorded near the time of these eruptions has been superimposed for scale. The blank region surrounding it corresponds to an occulting disk in one of SOHO's coronagraphic cameras. Speeding outward at millions of kilometers per hour, these two CMEs missed our fair planet. But those that do impact Earth's magnetosphere often trigger auroral displays and disruptions.

APOD: 2002 May 10 - Trailing Planets
Explanation: Positioning his camera and tripod on planet Earth, near Maricopa, Arizona, USA, astrophotographer Joe Orman created this trailing display of the ongoing sky-full-of-planets on May 3rd. He initially captured the grouping in a 20 second long time exposure recording the positions of the bright planets and stars. Covering the camera lens for five minutes, he then exposed the same frame for 45 minutes, tracing the gentle arcs of the celestial wanderers as the Earth's rotation carried them toward the western horizon. Of course these planets, Mercury, Venus, Mars, Jupiter, and Saturn all still dazzle in western skies near sunset, but sky gazers who want to see Mercury should look soon. Mercury starts the evening closest to the horizon - visible here above the wide bright trail left by Venus - and in the coming days Mercury will be the first to leave the evening sky entirely as it moves closer to the setting Sun. Tonight Venus and Mars will appear very close together, separated by only one third of a degree.

APOD: 2002 May 9 - Planets Over Stonehenge
Explanation: Stonehenge, four thousand year old monument to the Sun, provides an appropriate setting for this delightful snapshot of the Sun's children gathering in planet Earth's sky. While the massive stone structure dates from around 2000 B.C., this arrangement of the visible planets was recorded only a few days ago on the evening of May 4th, 2002 A.D. Bright Jupiter stands highest above the horizon at the upper left. A remarkable, almost equilateral triangle formed by Saturn (left), Mars (top), and Venus (right) is placed just above the stones near picture center. Fighting the glow of the setting sun, Mercury can be spotted closest to the horizon, below and right of the planetary triad. Still easy to enjoy for casual sky gazers, this photogenic and slowly shifting planetary grouping will be joined by a young crescent Moon beginning Monday, May 13.

APOD: 2002 May 8 - Sunspot Loops in Ultraviolet
Explanation: It was a quiet day on the Sun. The above image shows, however, that even during off days the Sun's surface is a busy place. Shown in ultraviolet light, the relatively cool dark regions have temperatures of thousands of degrees Celsius. Large sunspot group AR 9169 is visible as the bright area near the horizon. The bright glowing gas flowing around the sunspots has a temperature of over one million degrees Celsius. The reason for the high temperatures is unknown but thought to be related to the rapidly changing magnetic field loops that channel solar plasma. Sunspot group AR 9169 moved across the Sun during 2000 September and decayed in a few weeks.

APOD: 2002 April 4 - Ikeya-Zhang: Comet Over Colorado
Explanation: Comet Ikeya-Zhang ("ee-KAY-uh JONG") has become a most photogenic comet. This lovely early evening view of the comet in Rocky Mountain skies looks northwest over ridges and low clouds. The time exposure was recorded on March 31st from an 8,000 foot elevation near Yampa, Colorado, USA. Sporting a sweeping yellowish dust tail and blue ion tail eight to ten degrees long, Ikeya-Zhang is nestled near the horizon in the northern constellation of Andromeda. To the comet's left is the bright star Mirach or Beta Andromedae while the stretched celestial fuzzball to the comet's right is M31 or the Andromeda galaxy, the nearest bright spiral galaxy to our own Milky Way. As the days pass, Comet Ikeya-Zhang's apparent motion through the sky is towards the right in this image. Tonight, comet-watchers blessed with clear skies should find Ikeya-Zhang posing perfectly for binoculars and cameras just above M31, less than two degrees from the center of the bright galaxy.

APOD: 2002 March 10 - A Southern Sky View
Explanation: On 1996 March 22, a Galaxy and a comet shared the southern sky. They were captured together, from horizon to horizon, in the night sky above Loomberah, New South Wales, Australia by astronomer Gordon Garradd. Garradd used a home made all-sky camera with a fisheye lens, resulting in a circular 200 degree field of view. This gorgeous sky view was dominated by the luminous band of our Milky Way Galaxy cut by dramatic, dark interstellar dust clouds. Along with the bright stars of our Galaxy, the Large Magellanic Cloud is visible at the lower left. That night sky was also graced by the long, lovely, bluish tail of Comet Hyakutake, which can be seen toward the top of the image, near the bright star Arcturus. Bright city lights from nearby Tamworth glow along the Northwestern horizon.

APOD: 2002 February 27 - A Cloud Shadow Sunrise
Explanation: What could cause a ray of dark? Such a ray was caught in spectacular fashion above the Florida Everglades two years ago. The cause is something surprisingly familiar: a shadow. The gold-tinged cloud near the horizon blocks sunlight from reflecting off air behind the cloud, making that column of air appear unusually dark. Cloud shadows can be thought of the inverse of the more commonly highlighted crepuscular rays, where sunlight pours though cloud holes. Another seemingly opposite phenomenon, a sun pillar, involves small ice crystals floating high in the atmosphere.

APOD: 2002 February 21 - Comet Ikeya-Zhang
Explanation: Comet Ikeya-Zhang is presently heading north in planet Earth's sky, framed by stars of the constellation Cetus. The comet was discovered as a faint, telescopic object near the western horizon on the evening of February 1st independently by Kaoru Ikeya of Shizuoka prefecture, Japan, Daqing Zhang in Henan province, China, and later by observer Paulo Raymundo of Salvador, Brazil. But Ikeya-Zhang is expected to brighten significantly and in March and April could become visible to the unaided eye. This picture, taken near Tucson, Arizona, USA on the evening of February 9th, covers a field a bit less than the width of the full moon showing the comet's condensed coma and narrow, developing tail. Ikeya-Zhang should pass closest to the Sun (perihelion) on March 18 at a point roughly midway between the orbits of Mercury and Venus. Based on preliminary calculations of this comet's orbit, Ikeya-Zhang is suspected of being a periodic comet, returning to the inner Solar System every 500 years or so. In fact, it is speculated that Ikeya-Zhang may be directly connected with a historic bright comet seen in 1532.

APOD: 2002 February 9 - Moon Over Mongolia
Explanation: Fighting clouds and the glow of city lights, a young Moon shines over the western horizon of Mongolia's capital, Ulaan-Baatar. The thin sunlit crescent is about 2 days old and strongly over exposed in this image taken on March 10, 1997. The night side of the Moon is also visible due to earthshine - sunlight reflected from the Earth to the Moon. Just below the Moon, bright Saturn shines through the clouds. Skygazers will have a chance to watch the Moon actually pass in front of the ringed planet in February, March, and April this year. In fact, an excellent lunar occultation of Saturn will be visible from parts of North America on February 20th as Saturn disappears behind the dark limb of a first quarter Moon. Some may even take this opportunity to search for Saturn's lost ring.

APOD: 2002 January 30 - Moonrise Over Seattle
Explanation: Is the Moon larger when near the horizon? No -- as shown above, the Moon appears to be very nearly the same size no matter its location on the sky. Oddly, the cause or causes for the common Moon Illusion are still being debated. Two leading explanations both hinge on the illusion that foreground objects make a horizon Moon seem farther in the distance. The historically most popular explanation then holds that the mind interprets more distant objects as wider, while a more recent explanation adds that the distance illusion may actually make the eye focus differently. Either way, the angular diameter of the Moon is always about 0.5 degrees. In the above time-lapse sequence taken near the end of last year, the Moon was briefly re-imaged every 2.5 minutes, with the last exposure of longer duration to bring up a magnificent panorama of the city of Seattle.

APOD: 2002 January 9 - Blue Flash
Explanation: Difficult to observe, the momentary green flash above the rising or setting sun has been documented as a phenomenon caused by the atmospheric bending or refraction of sunlight. Like a weak prism, the Earth's atmosphere breaks white sunlight into colors, bending red colors slightly and green and blue colors through increasingly larger angles. When the sky is clear, a green flash just above the sun's edge can sometimes be seen for a second or so, when the sun is close to a distant horizon. A blue flash is even harder to see though, because the atmosphere must be extraordinarily clear to avoid scattering and diminishing the refracted blue sunlight. Still, from a site near Roques de los Muchachos (altitude 2,400 meters) on La Palma in the Canary Islands, astrophotographer Mario Cogo captured this dramatic telescopic image of a blue flash on color film in October of 2001. The image of the setting Sun with large sunspot groups on its surface is heavily distorted by atmospheric layers. A lingering green rim is just visible under the tantalizing blue flash.

APOD: 2001 December 26 - Himalayan Horizon From Space
Explanation: This stunning aerial view shows the rugged snow covered peaks of a Himalayan mountain range in Nepal. The seventh-highest peak on the planet, Dhaulagiri, is the high point on the horizon at the left while in the foreground lies the southern Tibetan Plateau of China. But, contrary to appearances, this picture wasn't taken from an airliner cruising at 30,000 feet. Instead it was taken with a 35mm camera and telephoto lens by the Expedition 1 crew aboard the International Space Station -- orbiting 200 nautical miles above the Earth. The Himalayan mountains were created by crustal plate tectonics on planet Earth some 70 million years ago, as the Indian plate began a collision with the Eurasian plate. Himalayan uplift still continues today at a rate of a few millimeters per year.

APOD: 2001 December 9 - The Belt of Venus
Explanation: Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink. Visible in the above photograph, this off-color band between the dark eclipsed sky and the blue sky can best be seen in the direction opposite the Sun and is called the Belt of Venus. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. The Belt of Venus can be seen from any location with a clear horizon. It is frequently caught by accident in other photographs.

APOD: 2001 October 30 - Anticrepuscular Rays Over Colorado
Explanation: What's happening over the horizon? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun and some well placed clouds. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Pictured above is a particularly striking set of anticrepuscular rays photographed earlier this month from a moving car just outside of Boulder, Colorado, USA.

APOD: 2001 September 12 - Zodiacal Light and the False Dawn
Explanation: An unusual triangle of light will be particularly bright near the eastern horizon before sunrise during the next two months for observers in Earth's northern hemisphere. Once considered a false dawn, this triangle of light is actually Zodiacal Light, light reflected from interplanetary dust particles. The triangle is clearly visible on the left of the above frame taken from Mauna Kea in Hawaii on August 30 by one of the developing global network of fisheye nighttime web cameras called CONCAMs. Zodiacal dust orbits the Sun predominantly in the same plane as the planets: the ecliptic. Indeed, the triangle points to bright spots Jupiter and Saturn, with Saturn nearer the center. Zodiacal light is so bright this time of year because the dust band is oriented nearly vertical at sunrise, so that the thick air near the horizon does not block out relatively bright reflecting dust. Zodiacal light is also bright for people in Earth's northern hemisphere in March and April just after sunset.

APOD: 2001 August 18 - Phobos: Doomed Moon of Mars
Explanation: Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1978 Viking 1 orbiter image, the largest moon, Phobos, is indeed seen to be a heavily cratered asteroid-like object. About 17 miles across, Phobos really zips through the Martian sky. Actually rising above Mars' western horizon and setting in the east, it completes an orbit in less than 8 hours. But Phobos is doomed. Phobos orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: 2001 August 7 - A July Dawn
Explanation: Those up before dawn in late July in the northern hemisphere could see planets, stars, and a spacecraft in a single quick glance before starting their day. Near the eastern horizon was bright Jupiter, and not far above and to its right was the very bright Venus. Connecting the dots will point you just right of Saturn. Far in the distance but near the top right of the frame is the Pleiades star cluster. Orbiting the Earth well in the foreground, the International Space Station reflected sunlight to cause the faint line segment. In the very close foreground, the bright red and yellow lines were caused by a passing van. The above picture was taken on July 26 from Quebec, Canada. Why are bushes visible through the van? The van was present for only a few of the 25 seconds of the total exposure.

APOD: 2001 June 27 - Moonlight, Mars and Milky Way
Explanation: Aloha and welcome to a breath-taking skyscape. In this celestial scene, a four day old Moon illuminates a dreamlike foreground while bright planet Mars (above center) rules and the Milky Way's cosmic clouds of stars and dust seem to stretch from horizon to horizon. The picture was taken on May 27th from what may be the best amateur astronomy observing site on planet Earth, near the Mauna Kea, Hawai'i Visitor Center, 9,600 feet above sea level. Remarkable in the volcanic foreground are moonlit clouds and an "ahu hoku" - a star marker or star altar - built up of rocks topped with a white piece of coral gently glowing in the moonlight. Now near its closest approach in 13 years, Mars still lingers between the Milky Way constellations of Sagittarius and Scorpius. High above the horizon by midnight, the Red Planet is exceptionally well placed for earthdwellers to admire it. Astrophotographer Barney Magrath comments that this splendid sky view represents one of the joys of photography itself. When making the time exposure he did not realize that the ahu hoku would become such a beautiful element in his celestial composition.

APOD: 2001 June 1 - Venus' Evening Loop
Explanation: From September 2000 through March 2001, astronomer Tunc Tezel patiently photographed the planet Venus on 25 different dates as it wandered through the evening twilight. The pictures were taken from the same spot on the campus of the Middle East Technical University near Ankara, Turkey, and timed so that for each photo the Sun was 7 degrees below the horizon. Carefully registering and combining the pictures, he produced this composite image -- a stunning demonstration of Venus' grand looping sky motion during its recent stint as planet Earth's evening star. As indicated, the first picture, taken September 28, 2000, finds Venus close to the western horizon and drifting south (left) with the passing days. By December however, Venus was climbing well above the horizon after sunset and in January 2001 it reached its maximum apparent distance (elongation) from the Sun. March found Venus falling from the evening sky while moving rapidly north, finally appearing (far right) as a faint dot against the sunset glow on March 24. This month, Venus rises before dawn as the brilliant morning star.

APOD: 2001 May 12 - Shuttle Moon
Explanation: As a gorgeous full Moon rose above the eastern horizon on February 7, the Space Shuttle Atlantis streaked skyward towards an orbital rendezvous with the International Space Station. Watching from Orlando, Florida, about 60 miles west of the Kennedy Space Center launch site, photographer Tony DeVito captured this digital image, one of a series of pictures of the shuttle's fiery climb. While foreground street lights flickered on and a clear evening sky grew dark, the shuttle's path just grazed the bright lunar disk. On this mission, STS-98, Atlantis carried the U.S. Destiny laboratory module to be added to the expanding orbital outpost. Atlantis is currently scheduled to return to the space station next month.

APOD: 2001 March 30 - Equinox + 1
Explanation: Twice a year, at the Spring and Fall equinox, the Sun rises due east. In an emphatic demonstration of this celestial alignment, photographer Joe Orman recorded this inspiring image of the Sun rising exactly along the east-west oriented Western Canal, in Tempe, Arizona, USA. But he waited until March 21st, one day after the equinox, to photograph the striking view. Why was the rising Sun due east one day after the equinox? At Tempe's latitude the Sun rises at an angle, arcing southward as it climbs above the horizon. Because the distant mountains hide the true horizon, the Sun shifts slightly southward by the time it clears the mountain tops. Waiting 24 hours allowed the Sun to rise just north of east and arc back to an exactly eastern alignment for the photo. Orman also notes that this picture carries a special significance as we experience the maximum of the solar activity cycle. The electricity and telephone transmission lines along the canal symbolize power and communications grids which are most vulnerable to outbursts from the active Sun.

APOD: 2001 March 23 - Mir Flares Farewell
Explanation: Streaking low across the western horizon after sunset, the Russian Mir space station makes a final pass through the evening sky above the coastal city of Salvador, Brazil. In this 5 minute 20 second time exposure made with ASA 800 film and a wide-angle lens on March 19, setting stars leave short, almost vertical trails. A rapidly moving Mir travels horizontally, trailing toward the left (south) edge of the picture. Reflecting sunlight from low Earth orbit, the historic space station chanced to produce a "farewell" flare near the end of its visible track. As if in poignant response, the Hubble Space Telescope appeared in Brazilian skies within a minute after Mir's passage and also left a flare along a trail moving toward the top of the picture. Lights visible on the horizon are from nearby Itaparica Island. After 15 years in service, the long-lived Mir space station was safely deorbited today. The splashdown of its surviving pieces occurred in a remote area of the South Pacific Ocean.

APOD: 2001 March 22 - Jupiter, Saturn and Messier 45
Explanation: Brilliant Venus falls out of the evening sky as March ends, but Jupiter and Saturn remain well up above the western horizon. Jupiter blazes forth above and to the left of a slightly fainter Saturn in this telephoto picture taken on January 19th. Near the top lies the lovely Pleiades star cluster with suggestions of its characteristic blue reflection nebulae. These planets and the Pleiades have a similar, easily recognizable orientation in the Spring night sky. Also known as M45, the 45th object in French astronomer Charles Messier's famous catalog, the Pleiades will likely soon be checked off many stargazers' tally lists. For northern hemisphere observers this weekend offers a prime opportunity to complete a Messier Marathon -- the viewing of all 110 Messier catalog objects in one glorious dusk to dawn observing run. This weekend it will also be possible to complete an all-planet marathon, observing all the solar system's planets in a single night. And if you still need something to look at, the International Space Station could also be visible arcing through the skies depending on your location, but Mir will not.

APOD: 2001 March 20 - Discovery Spring
Explanation: Welcome to the equinox! Moving northward in Earth's sky, today the Sun crosses the celestial equator at 13:31 Universal Time bringing Spring to the north and Fall to the south. The change of season is known as an equinox as the Sun rises due east on the horizon and sets due west -- providing an equal night, 12 night and 12 daylight hours, for both northern and southern hemispheres. In this picture from March 8, the Sun peers over the eastern horizon at the space shuttle Discovery's dramatic morning launch on mission STS-102. Having delivered supplies and taxied crew to the International Space Station, Discovery will remain in orbit for this first day of northern hemisphere Spring. Discovery is scheduled to land at Kennedy Space Center in Florida early tomorrow.

APOD: 2001 March 8 - Bright Venus
Explanation: Have you seen a bright evening star in the western sky lately? That's no star, that's planet Venus the second "rock" from the Sun. Blazing at -4.6 magnitude, Venus, after the Sun and Moon, is the third brightest celestial body in planet Earth's sky. Venus is closer to the Sun than Earth and as Venus orbits the Sun it is seen to go through phases similar to the Moon. But unlike the Moon, as Venus waxes and wanes its distance from Earth and hence its apparent size changes drastically. This causes Venus to look brighter as it looms large in its crescent phases than when it is smaller and nearly full. Taken on January 28th, this dramatic picture finds a crescent Venus near its brightest to the right of a crescent Moon. The brilliant rivals seem poised above a satellite dish of the Scripps Satellite Oceanography Facility. Closer to the horizon, just below and to the right of the satellite dish, Mercury pierces the twilight glow.

APOD: 2001 February 19 - Shuttle Plume Shadow Points to Moon
Explanation: Why would the shadow of a space shuttle launch plume point toward the Moon? Two weeks ago during the launch of Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Next, just at sunset, the shadow is the longest and extends all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon just happened to be.

APOD: 2001 February 10 - Aurora Astern
Explanation: Sailing upside down, 115 nautical miles above Earth, the crew of the Space Shuttle Endeavour made this spectacular time exposure of the southern aurora (aurora australis) in October of 1994. Aurora, also known as the northern and southern lights, appear as luminous bands or streamers of light which can extend to altitudes of 200 miles. They are typically visible from the Earth's surface at high latitudes and are triggered by high energy particles from the Sun. The delicate colors are caused by energetic electrons colliding with oxygen and nitrogen molecules in the upper atmosphere. In this picture, the rear structure of the shuttle Endeavour is in the foreground with the vertical tail fin pointed toward Earth. Star trails are the short streaks above Earth's horizon.

APOD: 2001 January 19 - Black Holes Are Black
Explanation: Q: Why are black holes black? A: Because they have an event horizon. The event horizon is that one-way boundary predicted by general relativity beyond which nothing, not even light, can return. X-ray astronomers using the space-based Chandra Observatory now believe they have direct evidence for event horizons - therefore black holes - in binary star systems which can be detected in x-ray light. These binaries, sometimes called x-ray novae, are known to consist of relatively normal stars dumping material on to massive, compact companions. As illustrated, the material swirls toward the companion in an accretion disk which itself glows in x-rays. If the compact companion is a neutron star (right), the material ultimately smashes into the solid surface and glows even more brightly in high energy x-rays. But if it is indeed a black hole with a defining event horizon, then the x-ray hot material approaches the speed of light as it swirls past the surface of no return and is lost from view. Recent work describes observations of two classes of x-ray binaries, one class 100 times fainter than the other. The results imply the presence of an event horizon in the fainter class which causes the extreme difference in x-ray brightness.

APOD: 2000 November 8 - October Skylights
Explanation: With brilliant Venus above the western horizon at sunset and Jupiter and Saturn high in the east by early evening, November's night sky is filled with bright planets. October's sky featured bright planets as well and, triggered by the active Sun, some lovely auroral displays. This colorful aurora was recorded by astrophotographer Wade Clark in skies above Hamilton, Washington, USA on the night of October 4th. Through the shimmering northern lights Jupiter and Saturn are easy to spot flanking the V-shaped head of Taurus the Bull. Of course, just above lies the lovely Pleiades star cluster. Solar activity will also produce auroral shows in November, particularly at high northern and southern latitudes. Plus, November skygazers can certainly anticipate a celestial performance on the evening of the 17th/18th -- the moonlit Leonid meteor shower.

APOD: 2000 September 12 - Slightly Above Mars Pathfinder
Explanation: If you could have hovered above the Pathfinder mission to Mars in 1997, this is what you might have seen. Directly below you is the control tower of Sagan Memorial Station. Three dark solar arrays extend out to collect valuable energy, surrounded by light-colored deflated airbags that protected Pathfinder's instruments from directly colliding with the rocky Martian surface. The left solar panel has ramps down which Pathfinder's rolling robot Sojourner started its adventure to nearby rocks. Sojourner itself is visible inspecting a rock nicknamed Yogi at 11 o'clock. Rocks cover the Martian surface, with Twin Peaks visible on the horizon at 9 0'clock. The distant sky is mostly orange. This image is a recently released digital combination of panoramic pictures taken by Pathfinder on Mars and a picture of a Lander scale model back on Earth. The Mars Pathfinder Mission was able to collect data for three months, sending back information that has indicated a wet distant past for Mars.

APOD: 2000 July 28 - Moon And Venus Share The Sky
Explanation: July is drawing to a close and in the past few days, some early morning risers could have looked east and seen a crescent Moon sharing the pre-dawn skies with planets Jupiter and Saturn. Planet Mercury will also pass about 2 degrees from the thin waning crescent Moon just before sunrise near the eastern horizon on Saturday, July 29. And finally, on the evening of July 31st, Venus will take its turn near the crescent Moon. But this time it will be a day-old crescent Moon near the western horizon, shortly after sunset. In fact, on July 31 (August 1 Universal Time) the Moon will occult (pass in front of) Venus for northwestern observers in North America. This telescopic picture taken on 31 December 1997, shows a lovely young crescent Moon and brilliant crescent Venus in the early evening sky near Bursa, Turkey.

And what about the Sun? On Sunday, July 30, a partial eclipse of the Sun will be visible from some locations in North America.

APOD: 2000 July 21 - Eros Craters And Boulders
Explanation: From a delicate orbit around asteroid 433 Eros, the NEAR Shoemaker spacecraft's camera has now imaged the entire surface of the small oddly-shaped world at least once. Recorded on July 7th from a distance of 50 kilometers, this dramatic view is about 1.8 kilometers across. It shows the walls and rims of two large overlapping impact craters on the horizon. Massive boulders which may be debris from the impacts are perched along the crater edges. The prominent boulder on the horizon near picture center is about 40 meters long. In fact, the NEAR mission to Eros has shown that along with craters and boulders, grooves and ridges are also common on the asteroid's surface. While the craters are clearly of impact origin, puzzles about the other surface features still remain. On July 13, controllers fired the spacecraft thrusters and moved NEAR Shoemaker to an even closer 35 kilometer orbit to enable higher resolution surface studies.

APOD: 2000 May 25 - Eros Horizon View
Explanation: Since April 30, the robotic NEAR-Shoemaker spacecraft has been orbiting only 31 miles from asteroid Eros. Cruising over the asteroid's north and south poles at a leisurely 7 miles per hour, the spacecraft completes an orbit once every 1.2 earth days. This dramatic horizon view recorded by the spacecraft's camera on May 18 spans about 0.8 miles and reveals features as small as 13 feet across. Emphasized by long, harsh shadows produced by the low sun angle, the rolling surface of Eros is seen to be strewn with boulders and craters with a range of sizes. The jagged-looking boulder near the picture center is over 190 feet tall. While gathering sharp pictures of Eros' surface, experimenters will also take advantage of the close orbit to explore the asteroid's surface composition and internal structure, and search for a magnetic field.

APOD: 2000 May 7 - A Green Flash from the Sun
Explanation: Many think it is just a myth. Others think it is true but its cause isn't known. Adventurers pride themselves on having seen it. It's a green flash from the Sun. The truth is the green flash does exist and its cause is well understood. Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green. The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. A green flash is also visible for a rising Sun, but takes better timing to spot. A slight variant of this was caught in the above photograph, where much of the Sun was still visible, but the very top appeared momentarily green. The Sun itself does not turn partly green, the effect is caused by layers of the Earth's atmosphere acting like a prism.

APOD: 2000 March 20 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it is never seen far from the Sun, and so is only visible near sunrise or sunset. If trailing the Sun, Mercury will be visible for several minutes before it follows the Sun behind the Earth. If leading the Sun, Mercury will be visible for only several minutes before the Sun rises and hides it with increasing glare. An informed skygazer can usually pick Mercury out of a dark horizon glow with little more than determination. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during the middle of last month. Each picture was taken from the same location in Spain when the Sun was 10 degrees below the horizon and superposed on the single most photogenic sunset.

APOD: 2000 February 13 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.

APOD: 2000 January 5 - Earth, Moon, Hubble
Explanation: The Space Shuttle Discovery Crew was fortunate enough to witness one of the brighter full moon's from orbit two weeks ago during their mission to fix the Hubble Space Telescope. Pictured on the left, the horizon of the Earth is visible below this full Moon, which is below the edge of the Hubble Space Telescope. The full Moon on this day, last December 22, was a few percent brighter than average because it was full at nearly the same time it was at its closest to the Earth, which comes at a time when the Earth is relatively close to the Sun. The Shuttle Crew successfully showered Hubble with needed holiday gifts, including six new gyroscopes, a new computer, and new batteries.

APOD: November 10, 1999 - The Belt of Venus
Explanation: Although you've surely seen it, you might not have noticed it. During a cloudless twilight, just before sunrise or after sunset, part of the atmosphere above the horizon appears slightly off-color, slightly pink. Visible in the above photograph, this off-color band between the dark eclipsed sky and the blue sky can best be seen in the direction opposite the Sun and is called the Belt of Venus. Straight above, blue sky is normal sunlight reflecting off the atmosphere. In the Belt of Venus, however, the atmosphere reflects light from the setting (or rising) Sun which appears more red. The Belt of Venus can be seen from any location with a clear horizon.

APOD: November 5, 1999 - Shadow Of Phobos
Explanation: Hurtling through space above the Red Planet, potato-shaped Phobos completes an orbit of Mars in less than eight hours. In fact, since its orbital period is shorter than the planet's rotation period, Mars-based observers see Phobos rise in the west and set in the east - traveling from horizon to horizon in about 5 1/2 hours. These three images from the Mars Global Surveyor (MGS) spacecraft record the oval shadow of Phobos racing over western Xanthe Terra on August 26, 1999. The area imaged is about 250 kilometers across and is seen in panels from left to right as red filter, blue filter, and combined color composite views from the MGS wide-angle camera system. The three dark spots most easily seen in the red image are likely small fields of dark sand dunes on crater floors. Standing in the shadow of Phobos, you would see the Martian version of a solar eclipse!

APOD: September 3, 1999 - Venus Falls Out of the Evening Sky
Explanation: Orbiting closer to the Sun than planet Earth, bright Venus always appears to be near the Sun's position in our sky and often shines near the horizon in twilight hours. In fact, after posing as the brilliant evening star for the first half of this year, Venus has now swung around its orbit and is emerging in the predawn twilight as the morning star. Taken during its stint as the evening star, this imaginative long-exposure photo, of Venus and a 2-day-old crescent Moon gives the illusion of the pair "falling out" of the western sky. After an initial short exposure captured the Moon and Venus, the lens was covered for a few minutes, then left uncovered to record the trails until the Moon had set.

APOD: July 14, 1999 - Moon, Planets, and Rocket Trails
Explanation: Are you an early riser? Over the last month or so, the bright planets Jupiter and Saturn have come to adorn eastern skies before sunrise. In fact, astrophotographer Joe Orman anticipated that an early bird's reward for looking east on June 10 would be this pleasing arrangement of Jupiter (top right), a crescent Moon, and Saturn (near center), but he was surprised to also find these eerie, iridescent clouds wafting through the pre-dawn sky over suburban Phoenix, Arizona, USA. The clouds turned out to be rocket engine trails from defense missile tests at the range in White Sands, New Mexico ... about 300 miles away. While the Moon's phase is just past new moon, gone now from the pre-dawn horizon, brilliant Jupiter and Saturn can still be seen high toward the southeast in the constellation Aries.

APOD: June 19, 1999 - Venus on the Horizon
Explanation: Venus can appear as a brilliant evening star. Besides the sun and moon, Venus is the brightest object visible in Earth's sky. Because it is closer to the sun than Earth, Venus never strays far from the sun in its apparent position and is seen during the year as either a bright morning or evening star. This beautiful sunset imaged from low earth orbit by the Atlantis space shuttle crew in May 1989 also reveals the planet Venus blazing above Earth's horizon. It is a fitting image for this mission and crew. It was recorded following the successful release of the robot Venus-explorer Magellan, the first planetary probe to be deployed from a space shuttle.

APOD: May 21, 1999 - Star Party Trails
Explanation: Stargazing is fun! If you'd like to try it, this weekend may be your chance as many astronomy clubs and organizations will be hosting public celebrations of Astronomy Day on Saturday, May 22nd. In recent years, open house nights at observatories, astronomy club gatherings, and star parties have become increasingly popular. They offer great opportunities for beginners to view the sky through a variety of telescopes and veterans to swap stories and ideas. This time exposure of star trails was made last month at the Sentinel, Arizona "Stargaze" star party. On the right, a brilliant trail tracks the setting evening star, Venus. Stars in Orion are near the center and bright Sirius produced the prominent trail at the left. City lights from nearby Yuma glow on the horizon while party-goers' red filtered flashlights create the eerie foreground effect. The red flashlights are courteously used to provide a safe level of illumination but still preserve night vision for enjoyable stargazing.

APOD: March 25, 1999 - March of the Planets
Explanation: This March stargazers have been treated to eye-catching formations of bright planets in western evening skies. On March 3rd, looking toward a beautiful sunset from a beach on the Hawaiian isle of Maui, photographer Rick Scott recorded this fleeting, four-planet "hockey stick" array. Mercury, closest to the horizon and immersed in fading sunlight, is easily visible between silhouetted clouds. To the left and up in the deepening blue is Jupiter with a brilliant Venus above and Saturn shining in the darkened sky near the top of the image. The planets are seen to lie close to the ecliptic - the apparent path of the sun - which is nearly perpendicular to the horizon for Hawaiian latitudes at this time of year.

APOD: March 20, 1999 - Aurora and Orion
Explanation: Looking toward the south from low Earth orbit, the crew of the Space Shuttle Endeavor made this stunning time exposure of the Aurora Australis or southern lights in April of 1994. Aurora are visible at high northern latitudes as well, with the northern lights known as Aurora Borealis. They are caused by high energy electrons from the Solar Wind which are funneled into the atmosphere near the poles by the Earth's magnetic field. The reddish colors occur at the highest altitudes (about 200 miles) where the air is least dense. At lower altitudes and greater densities green tends to dominate ranging to a pinkish glow at the lowest. The familiar constellation of Orion the Hunter is clearly visible above the dark horizon in the background. Because of the shuttle's orbital motion, the bright stars in Orion appear slightly elongated.

APOD: February 26, 1998 - A Southern Sky View
Explanation: From horizon to horizon, the night sky above Loomberah, New South Wales, Australia was photographed by astronomer Gordon Garradd on March 22, 1996. Garradd used a home made all-sky camera with a fish-eye lens, resulting in a circular 200 degree field of view. This gorgeous sky view is dominated by the luminous band of the Milky Way cut by dramatic, dark interstellar dust clouds. Along with the bright stars of our Galaxy, the Large Magellanic Cloud is visible at the upper right (about 1 o'clock) and the long, lovely, bluish tail of comet Hyakutake can be seen toward the bottom of the image, near the bright star Arcturus. Bright city lights from nearby Tamworth glow along the Northwestern horizon.

APOD: December 4, 1997 - A Sky Full Of Planets
Explanation: Look up tonight. Just after sunset, the crescent moon and all five "naked-eye" planets (Mercury, Venus, Mars, Jupiter, and Saturn) will be visible (depending on your latitude), lying near our solar system's ecliptic plane. Venus and Jupiter will shine brilliantly as the brightest "stars" in the sky, but Mercury will be near the horizon and hard to see. A pair of binoculars will also reveal Uranus and Neptune and observers with a telescope and a good site may even be able to glimpse faint Pluto just above the Western horizon in the fading twilight (not shown on the chart above). Enjoy this lovely spectacle any clear night through about December 8. A similar gathering is expected in May 2000 but the planets will be hidden from view by the solar glare. A night sky as full of planets as this one will occur again though ... in about 100 years.

APOD: November 16, 1997 - The Leonid Meteor Shower
Explanation: The Leonid Meteor Shower will likely reach its peak in the early hours this Monday morning. Though the Moon will be bright, Leo, the shower's radiant point, will be well above the eastern horizon from Western North America and the Pacific region during this period. This year's Leonids may prove particularly exciting as observers anticipate the legendary Leonid storm of activity will occur sometime during the next few apparitions of this annual meteor shower - although most expect the meteor storm to occur in 1998 or 1999. Meteor showers result from debris left by passing comets. The Leonids specifically are small pieces of Comet Tempel-Tuttle. In the above series of time-lapse, 1-minute exposures, a 1995 Leonid is seen to leave a train of hot air that glowed persistently for several minutes.

APOD: October 14, 1997 - Venus On The Horizon
Explanation: The month of October features a sky full of planets, including Venus as the brilliant evening star. Besides the sun and moon, Venus is the brightest object visible in Earth's sky. This month, Venus appears in early evening near the red planet Mars and Mars' red giant rival Antares above the southwestern horizon. Because it is closer to the sun than Earth, Venus never strays far from the sun in its apparent position and is seen during the year as either a bright morning or evening star. This beautiful sunset imaged from low earth orbit by the Atlantis space shuttle crew in May 1989 also reveals the planet Venus blazing above Earth's horizon. It is a fitting image for this mission and crew. It was recorded following the successful release of the robot Venus-explorer Magellan, the first planetary probe to be deployed from a space shuttle.

APOD: September 16, 1997 - Moon Over Mongolia
Explanation: Fighting clouds and the glow of city lights, a young Moon shines over the western horizon of Mongolia's capital Ulaan-Baatar. The thin sunlit crescent is 1.5 days old and strongly over exposed in this image taken on March 10. The night side of the moon is also visible due to Earthshine - sunlight reflected from the Earth to the Moon. Just below the Moon, bright Saturn shines through the clouds. Early morning risers can see the Moon near Saturn this week. In fact, from North America the Moon can be seen to pass in front of the ringed planet on Thursday morning, September 18th.

APOD: September 9, 1997 - A Green Flash from the Sun
Explanation: Many think it is just a myth. Others think it is true but its cause isn't known. Adventurers pride themselves on having seen it. It's a green flash from the Sun. The truth is the green flash does exist and its cause is well understood. Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green. The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. A green flash is also visible for a rising Sun, but takes better timing to spot. A slight variant of this was caught in the above photograph, where much of the Sun was still visible, but the very top appeared momentarily green. The Sun itself does not turn partly green, the effect is caused by layers of the Earth's atmosphere acting like a prism.

APOD: August 25, 1997 - A Fisheye View of Comet Hale-Bopp
Explanation: Thousands of stars, several constellations, a planet and a comet all graced the western horizon over Ujue, Spain just after sunset on April 4th, 1997. Because the picture was taken with a fisheye lens, much of the whole night sky is visible. Comet Hale-Bopp, with both tails blazing, appears right of center. The brightest star is Sirius near the edge, well to the left of the constellation Orion. The red star above the belt of Orion is Betelgeuse, while the red star near the center is Aldebaran, just to the left of the bright Pleaides star cluster. Many other interesting astronomical objects are visible, including zodiacal light, which is the diffuse triangular glow in the center. Even the planet Mercury appears just over the horizon.

APOD: July 24, 1997 - Mars Pathfinder's Landing Site
Explanation: Where is Mars Pathfinder? Follow the arrow in the above picture taken by the Viking Orbiter in 1976. From the surface Mars appears covered with rocks, but from orbit Mars appears covered with craters. However, several familiar features are visible in this photograph. To the left (west) of Sagan Memorial Station are the now-familiar twin peaks that dominate the horizon of many Pathfinder photographs. These hills are about one kilometer from the landing site, twice the planned range of Sojourner. Two craters loom nearby: a small one to the east not easily visible here, and a big one to the south of Pathfinder. The landing site is in the dry flood channel named Ares Vallis.

APOD: July 14, 1997 - Mars: Twin Peaks In Stereo
Explanation: Get out your red/blue glasses and gaze across the surface of Mars in stereo. You are looking south of west across an ancient flood channel, Ares Vallis, landing site of the Mars Pathfinder. A rover deployment ramp and lander petal define the foreground in this 3D-view while a field of rocks seems to stretch to the horizon. At the upper right, over half a mile distant, are the hills known as the "Twin Peaks". Today is Sol 10, the tenth day the Pathfinder lander and Sojourner rover have been operating on the martian surface. Over that period, the mission has been returning a wealth of images and data. The otherwise successful rover activities have been recently hampered by some communication and computer difficulties.

APOD: January 26, 1997 - Aurora and Orion
Explanation: Looking toward the south from low Earth orbit, the crew of the Space Shuttle Endeavor made this stunning time exposure of the Aurora Australis or southern lights in April of 1994. Aurora are visible at high northern latitudes as well, with the northern lights known as Aurora Borealis. They are caused by high energy electrons from the Solar Wind which are funneled into the atmosphere near the poles by the Earth's magnetic field. The reddish colors occur at the highest altitudes (about 200 miles) where the air is least dense. At lower altitudes and greater densities green tends to dominate ranging to a pinkish glow at the lowest. The familiar constellation of Orion the Hunter is clearly visible above the dark horizon in the background. Because of the shuttle's orbital motion, the bright stars in Orion appear slightly elongated.

APOD: January 15, 1997 - Black Hole Signature From Advective Disks
Explanation: What does a black hole look like? If alone, a black hole would indeed appear quite black, but many black hole candidates are part of binary star systems. So how does a black hole binary system look different from a neutron star binary system? The above drawings indicate it is difficult to tell! Recent theoretical work, however, has provided a new way to tell them apart: advective accretion flows (ADAFs). A black hole system so equipped would appear much darker than a similar neutron star system. The difference is caused by the hot gas from the ADAF disk falling through the event horizon of the black hole and disappearing - gas that would have emitted much light were the central object only a neutron star. Recent observations of the soft X-ray transient V404 Cyg has yielded a spectrum much like an ADAF onto a black hole - and perhaps brighter than allowable from an ADAF onto a neutron star.

APOD: November 30, 1996 - Aurora Astern
Explanation: Sailing upside down, 115 nautical miles above Earth, the crew of the Space Shuttle Endeavour made this spectacular time exposure of the southern aurora (aurora australis) in October of 1994. The aurora, also known as the northern and southern lights, appear as luminous bands or streamers of light which can extend to altitudes of 200 miles. They are typically visible from the Earth's surface at high latitudes and are caused by high energy particles from the Sun. The delicate colors are caused by energetic electrons colliding with oxygen and nitrogen in the atmosphere. In this picture, the rear structure of the Space Shuttle is visible in the foreground with the vertical tail fin pointed toward Earth. Star trails are visible as small streaks above Earth's horizon.

APOD: October 13, 1996 - The Earth Also Rises
Explanation: The Lunar Orbiter 1 spacecraft was launched in 1966 to map the lunar surface in preparation for the Apollo moon landings. NASA's plucky robotic explorer performed its job well and pioneered this classic view of the Earth poised above the lunar horizon. The first humans to directly witness a similar scene were the Apollo 8 astronauts. As they orbited the Moon in December of 1968 they also recorded Earth rise in a photograph that was to become one of the most famous images in history - a moving portrait of our world from deep space.

APOD: September 13, 1996 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercuryrotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.

APOD: April 23, 1996 - Comet Hyakutake on a Starry Night
Explanation: It was a starry night in April (April 9th, 1996, 9:32 pm CDT to be exact) near Lone Jack, Missouri when Comet Hyakutake graced this astronomically rich field. Making an appearance as the brilliant evening star, Venus is overexposed at the far left. Just below Venus and slightly to the right, the Pleiades star cluster (M45) glistens. On the right hand side of the image, the comet itself shows a bright blue tail extending upwards past the nearby star cluster in the constellation Perseus (top right, the Alpha Persei Group). Hyakutake, receding from the Earth and appoaching the Sun, will sink into the western horizon at sunset in late April, disappearing from Northern Hemisphere skies.

APOD: April 18, 1996 - Hyakutake, Venus, Orion, and Pond
Explanation: Can you find Comet Hyakutake in the above picture? In this gorgeous photo, the starry night sky of April 9th is pictured with its new comet visitor. In the foreground is a pond with the lights of Kansas City, Missouri on the western horizon. On the upper left, the constellation of Orion is visible. At the center, the brightest object in the picture is the planet Venus. Venus's reflection can be seen in the pond. On the right - halfway between Venus and the photograph's edge - can be seen two bright objects fairly close to each other. Of these two, look closely at lower right object. See the tail? Comet Hyakutake is still visible for Northern observers in the Western sky and now has begun to brighten again as it nears the Sun.

APOD: November 14, 1995 - Aurora and Orion
Explanation: Looking toward the south from low Earth orbit, the crew of the Space Shuttle Endeavor made this stunning time exposure of the Aurora Australis (southern lights) in April of 1994. The aurora are caused by high energy electrons from the Solar Wind which are funneled into the atmosphere by the Earth's magnetic field. The reddish colors occur at the highest altitudes (about 200 miles) where the air is least dense. At lower altitudes and greater densities green tends to dominate. At the lowest altitudes a pinkish glow is sometimes produced. The familiar constellation of Orion the Hunter is clearly visible above the dark horizon in the background. Because of the shuttle's orbital motion, the bright stars in Orion appear slightly elongated.

APOD: October 26, 1995 - Aurora Astern
Explanation: Sailing upside down, 115 nautical miles above Earth, the crew of the Space Shuttle Endeavour made this spectacular time exposure of the southern aurora (aurora australis) in October of 1994. The aurora, also known as the northern and southern lights, appear as luminous bands or streamers of light which can extend to altitudes of 200 miles. They are typically visible from the Earth's surface at high latitudes and are caused by high energy particles from the Sun. The delicate colors are caused by energetic electrons colliding with oxygen and nitrogen in the atmosphere. In this picture, the rear structure of the Space Shuttle is visible in the foreground with the vertical tail fin pointed toward Earth. Star trails are visible as small streaks above Earth's horizon.