Interplanetary Fall

Today our planet joins two other worlds in the solar system where it is northern autumn.

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Won't be long 'til summer time is through...
The Beach Boys, "All Summer Long"

September 22, 2000 -- Every year around this time thousands of penguins rejoice to see the Sun peep above the Antarctic horizon. The return of sunlight after nearly 6 months of chilly darkness means it's time to shed a few pounds of blubber, find a mate, and bask in the sunshine. Spring is in the air.

On the other end of the world, sun-dappled puddles of water are freezing at the north pole, while Arctic bears are pondering hibernation. The north polar Sun, circling downward on a horizon-skirting 360 degree spiral, will soon be gone.

What powerful force of nature can rouse the passions of penguins at one end of the world and put mighty bears to sleep at the other? It's the changing of the seasons from northern summer to fall-- a special date on the calendar that northerners call the autumnal equinox.

Above: This is an image of the Automated Astrophysical Site-Testing Observatory near the US South Pole Station captured less than one day before the 2000 September equinox. The soft-yellow dawn sky presages the coming of a 6 month-long day. Click for images updated every 10 minutes.

For the past six months our planet's north pole has been tilted toward the Sun, most directly on June 21st, which was the beginning of northern summer. But, as The Beach Boys pointed out in a popular tune, there's no such thing as an Endless Summer. Today (Sept. 22nd) at 1727 UT (1:27 pm EDT) our planet's "subsolar point" crossed the equator heading south. With it, spring began in the southern hemisphere and autumn in the north.

On days when the Sun is shining directly over the equator, daylight and darkness are of nearly equal length. The word equinox comes from a Latin word meaning "equal night."

Left: The red dot marks Earth's subsolar point (the location where the Sun is directly overhead) at noon Eastern Standard Time throughout the year. Equinoxes occur when the subsolar point crosses the equator, once in March (the Vernal Equinox) and again in September (the Autumnal Equinox). This animation is based on images generated by JPL's Solar System Simulator.

Contrary to the all-too-popular notion that Earth is closer to the Sun during summer and farther away during winter, seasons are not caused by the eccentricity of our planet's orbit. Indeed, during the hottest days of northern summer the Earth is at its greatest distance from the Sun.

The primary cause of seasonal extremes on Earth is the 23 degree tilt of our planet's spin axis. When the north pole is tilted toward the Sun, northern days are long and the weather is warm. Six months later, as the south pole tilts toward the Sun, the southern hemisphere takes its turn at summer.

Seasons in the two hemispheres are always reversed. When it is summer in New York, it is winter in Sydney. On a spring day in Paris, autumn leaves are falling in Argentina.

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With the coming of fall in Earth's northern hemisphere, our planet joins two other worlds where it is also northern autumn: Saturn and Neptune. Fall on those planets isn't much like Earth. There are no golden leaves that fall to the ground or gently chilled breezes that hint of the coming winter. Saturn and Neptune don't even have a "ground" for leaves to land on. Both are giant gaseous balls of hydrogen and helium. The "gentle" chill of 1000 km-per-hour autumnal winds on Saturn registers -130 C. That's balmy compared to Neptune, where temperatures are 100 degrees lower and the winds blow nearly as hard.

While seasons on other planets may seem alien, they are defined just as they are on Earth. When the Sun shines down directly over a planet's equator -- that's an equinox. When one of the poles is tilted toward the Sun to its maximum extent -- that's a solstice. The equinoxes and solstices for 8 of the 9 planets are tabulated below. (Pluto is omitted, because we know so little about that distant world.)

Alien Seasons

The unfamiliar seasons of other worlds would strike any Earth-dweller as strange -- but some planets are more alien than others.

Take Mars for example. Mars has the highest orbital eccentricity of any planet except Mercury and Pluto. Its distance from the Sun varies between 1.64 and 1.36 AU over the Martian year. This large variation, combined with an axial tilt greater than Earth's, gives rise to a very peculiar seasonal change. When Mars is closest to the Sun, it is wintertime at the planet's north pole. Bone-chilling temperatures at that end of the planet plunge so low that carbon dioxide -- the main constituent of Mars's atmosphere -- freezes and falls to the ground. So much of the red planet's atmosphere turns to ice, that the global atmospheric pressure drops by 25%! Mars's atmosphere is noticeably thinner during northern winter.

Martian seasons are peculiar by Earth-standards, but they pale in comparison to seasons on Uranus. Like Earth, Uranus follows an orbit that is nearly circular; it keeps the same distance from the Sun throughout its long year. But, Uranus's spin axis is tilted by a whopping 82 degrees! This gives rise to extreme 20-year-long seasons and unusual weather. For nearly a quarter of the Uranian year (equal to 84 Earth years), the Sun shines directly over each pole, leaving the other half of the planet enveloped by darkness.

Left: This dramatic time-lapse movie captured by NASA's Hubble Space Telescope shows seasonal changes on Uranus. Once considered one of the blander-looking planets, Uranus is now revealed as a dynamic world with the brightest clouds in the outer Solar System. more info.

The northern hemisphere of Uranus is just now coming out of the grip of its decades-long winter. Sunlight, reaching some latitudes for the first time in years, warms the atmosphere and triggers gigantic springtime storms comparable in size to North America with temperatures of 300 degrees below zero. In the animation pictured left, bright clouds are probably made of crystals of methane, which condense as warm bubbles of gas percolate upwards through the atmosphere.

Mercury's seasons -- if they can be called that -- may be the most remarkable of all. Mercury rotates three times on its spin axis for each two orbits around the Sun. It is the only one of our solar system's planets or moons tidally locked in an orbital-to-rotational resonance with a ratio other than 1:1.

Mercury's weird rotation combined with the high eccentricity of its orbit would produce some very strange effects for an observer on that planet. At some longitudes, Mercurian skywatchers would see the Sun rise and gradually increase in apparent size as it slowly moved toward the zenith (i.e., the solstice!). At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon, all the while decreasing in apparent size. Observers at other points on Mercury's surface would see different but equally bizarre motions.

Temperature variations on Mercury, ranging from -200 C at night (winter) to 400 C during the day (summer), are the most extreme of any world we know.

Summer on Mercury is hot, but it's not the hottest spot in the solar system. The surface of Venus, warmed by a runaway greenhouse effect in Venus's carbon dioxide atmosphere, is even hotter at 470 C -- easily hot enough to melt lead. Because of Venus's thick cloud cover and circular orbit, seasonal changes in temperature are likely to be small. Indeed, of all the worlds in the solar system, ever-hot Venus might be the one with a truly Endless Summer. On the other hand, its choking acid-rich atmosphere and perpetual cloud cover would probably dampen enthusiasm for sun bathing.

There's no doubt that extraterrestrial seasons are harsh and uncomfortable. So, while you're lamenting the prospect of raking autumn leaves weekend after weekend for the next two months, take a quick mental tour of the solar system. Yard work on Earth might not seem so bad after all!

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Authors: Dr. Tony Phillips Production Editor: Dr. Tony Phillips Media Contact: Steve Roy Curator: Bryan Walls Responsible NASA official: Ron Koczor