Because of its proximity (mean distance: 385,000 km, ~238,000 miles from Earth), the Moon has always been the "heavenly body" most easily and ambitiously studied. The space program made the lunar surface an early target, with a number of exploratory, mainly photographic missions and culminating with six manned landings during the Apollo Program. On this page some of the characteristics of the Moon, its albedo, the phases of illumination, and its orbital "locking" on Earth are described. Then, some of the remote sensing studies, optical and radar, of the lunar surface are considered.
Lets begin with an image that places the Moon in context with its parent planet Earth. This view, taken from the Galileo spacecraft to be described later, is what an extraterrestrial space traveler might see as he/she/it approaches the Earth-Moon system.
Without question, the greatest part
of early remote sensing investigations of the planets, as expected from its
proximity, centered on the Moon. Indeed, its mean distance from Earth of 384,400 km (238,710 miles) meant that even before the 20th Century, dreamers could fancy someday going there for a visit (Best known fantasy: Jules Verne's Journey to the Moon). Earth's sole satellite is much smaller than the parent body: 3475 km (2160 miles) in diameter. (For a quick synopsis of the Moon's general
characteristics, read through this review by Bill Arnett: The Moon). Then, also check with the Web Site Rosanna Hamilton's Moon Page, looking particularly at the Table of Lunar Statistics and the section on the origin of the Moon stemming from an early huge impact of an asteroid with the primitive Earth).
Lunar Statistics are important and informative. Reproduced here are most of the "vital" ones: * Mean Distance of Moon from Earth - 238,712 mi (384,400 km)
* Greatest Distance of Moon from Earth (Apogee) - 252,586 mi (406,740 km)
* Shortest Distance of Moon from Earth (Perigee) - 221,331 mi (356,410 km)
* Circumference - 6,790 mi. (10,930 km.) 0.27 of Earth's circumference
* Diameter - 2,160 mi. (3,476 km.) 0.27 of Earth's diameter
* Equatorial Radius - 1,079 mi. (1,738 km.)
* Polar Radius - 1,077 mi. (1,735 km.)
* Mean Angular Diameter - 31' 07" * Mass Ratio (Earth Moon) - 81.301
* Volume - 2.4 x 10(9) mi³ (2.197 x 1010km³
* Mean Density - 208 lb/ft³ (3.34 g/cm³) 0.6 Earth's density * Gravity at Surface - 5.31 ft/s² (1.62 m/s²) 1/6 Earth's gravity
* Escape Velocity - 1.48 miles/sec (2.38 km/sec)
* Mean Inclination to Lunar Equator - 6° 41'
* Mean Orbital Inclination to Ecliptic - 5° 08' 43"
* Oscillation of Orbital Inclination to Equator - +/- 0° 9' every 173 days
* Inclination of Lunar Equator to Ecliptic - 1° 32' 33"
* Period of Revolution of Perigee - 3,232 days
* Period of Rotation about the lunar axis - 27.3 days
* Orbital Direction - east (counterclockwise from above)
* Mean Orbital Speed - 2,287 mi/h (3,683 km/h) 33 minutes arc/hour
* Daily Sidereal Motion - 13.176358°
* Mean Centripetal Acceleration - 0.0003 g
* Mean Eccentricity of Orbit - 0.0549
* Synodic Month - 29.53059 days (29 days, 12 hr. 44 min, 2.8 sec.)
* Sidereal Month (star to star) - 27 days, 7hr. 43min, 11.5sec.
* Anomalistic Month (Apogee to Apogee) - 27 days, 13hr. 18Min, 33.2sec. * Rotational Period - 27 days, 7hr, 43min. 11.5sec
* Surface Temperature - day = 273 F (120° C) night = -244F (-153° C)
* Visible Surface - 41% during lunar cycle, additional 18% from librations
* Parallax - 0.9507° * Moon's Angular Diameter - 0.5181°
* Magnitude of Full Moon - -12.5
* Average Albedo - 0.07
* Estimated Age - 4.6 billion years
* Flight Time from Earth - 60 to 70 hours
* Increase in Mean Distance from Earth - 1.5 inches per year (3.8 cm per year) One of the above parameters reveals why we see the Moon in the first place. The mean orbital inclination to the ecliptic of just greater than 5° means that the Moon's orbit is offset enough so that the Sun is not routinely blocked by the Moon and the Moon is not so blocked by Earth that sunlight fails to hit it. At any time of the year, say the time when this paragraph was written on August 10 of 2006, the Sun arises for the day from a direction South of East, transits in an arc towards the sky's zenith, sets towards the West. (This path is maximum at the Summer Solstice and moves progressively lower towards a minimum height in the south part of the Celestial Sphere until Winter Solstice, then starts rising in the Sphere until the Summer Solstice). The position of the Sun at High Noon varies over time owing to the 365 day year that Earth takes to orbit (revolve) around the Sun (see page 20-2). The Moon follows a similar path during any same period but is displaced 5° to the west. The first Crescent Moon (see below) reaches above the southwestern horizon early at night. Each successive night is marked by the Moon having undergone a shift along its orbit of 13.18° towards the southeastern direction of the orbital arc; the Moon arises progressively later each new night (about 50 minutes) as it moves through its sequence of phases (described below). These solar-lunar patterns of arc transits will vary depending upon the latitude of the observer. The total intensity or brightness
of reflecting surfaces at specific points or areas varies with the phases
of the Moon (progressive waxing and waning due to illumination changes, depending on relative positions of the Sun, Earth, and Moon). The lunar photometric function,
a plot of intensity versus phase angle, represents the intensity mathematically.
The degree of polarization of light from the Moon also shifts with the phase
angle and can show considerable variations in different regions of the Moon,
again related to composition and textural changes. These two similar diagrams show how the observed phases are controlled by the relative positions of the Moon and Earth with respect to solar illumination.
From StarrySkies.com
The Moon's orbit is not circular, but is an ellipse or elongate oval. The furthest out point (410,452 km; 252,586 miles) in the orbit is called the lunar apogee. The closest point (359,663 km; 221,331 miles) is the lunar perigee. The Earth's orbit also has an aphelion relative to the Sun, which occurs in Summer, and a perihelion, which is in Winter. The closer approach of the Earth at perihelion to the Sun increases the amount of light received by the lunar face, making the Moon seem brighter in the winter (when the atmosphere tends to be clearer) and less so in summer at aphelion.
19-2: What are the vernacular names applied to the phases of the Moon? ANSWER
To reiterate: The Moon rises and sets in roughly the same places on the horizon as the Sun, but about 50 minutes later each night. Every 29-30 days there is a New Moon - we cannot see it because it is near the bright late afternoon Sun in the sky. A few days later we can see a thin Crescent Moon, with its limited light surface on its eastern flank, which appears high in the western sky soon after sunset. This first visible sliver of illuminated Moon (the boundary between light and dark is the terminator) was actually in the Celestial Sphere and was rising during the latter part of the day, but this could not been seen because of the blinding effect of daylight from the west. Each successive night, as its phase gets greater (called waxing), the Moon becomes seen further south along its transit arc and sets a bit later until about 14 days after the New Moon we see a Full Moon. The Sun and the Moon are on opposite sides of the Earth, and after the Sun sets, the Moon rises. A Full Moon usually arises during the evening hours and, depending on when dawn occurs, may be visible much of the night. For the next 14 days the Moon's phases wane back to a crescent on its West which rises shortly ahead of the Sun, before the whole process repeats. In the summer's long daylight, a largely illuminated Moon can often be seen well into the sky in daylight. The entire sequence of phases is called a lunation. The amount of an illuminated Moon, at any phase, and the duration it is visible to a ground observer will change with the seasons, since in Winter darkness can last up to 14 hours and in high Summer it can be light beyond dawn for as much as 16 hours. And, the path actually followed by the Moon across the sky depends on where the observer lives - it is latitude-dependent.The first telephoto image of the Moon's surface
was made in the 1860s. Prior to the space program, almost all lunar studies used the
telescope. At first, emphasis was on optical observations (and photographs)
within the visible spectrum. Among the first measurements was the Moon's albedo,
the ratio (stated as a percentage) of the light reflected to the light incident
on a surface. (The Earth's albedo is 39% on average [with slight fluctuations
due to varying cloud cover] and Venus falls between 70% and 90% [from strong
reflections of sunlight by a thick planetwide cloud cover].) By comparing albedos
measured on possible terrestrial counterparts, the lunar albedo offers insights
into the composition, texture, and structure of its surface materials. Once established, the trick is to
interpret these albedo values.
We can measure the albedos of likely surface
materials in the laboratory, where we can vary the textures and illumination
conditions. In this way, lunar scientists postulated that the dark areas (maria, singular - mare) were some
type of volcanic material, most likely basalt. The lighter areas (terrae, singular terra, or highlands) were more
enigmatic. Several other volcanic types could produce albedos in the proper
range, but uncertainties about particle size and other variables inhibited definitive
identification. The first unmanned landers, the five Surveyors, resolved the
problem. The answer was that the highlands included a different group of rock
types, of which anorthosite was prevalent. Anorthosite is an igneous rock that contains large amounts
of light-colored feldspars.
Investigators judged that the albedo values for both the maria and highlands were consistent with scattering by a granular or fragmental loose surface covering having textures associated with unconsolidated surface materials (some thought a landed spacecraft would sink into loose, almost powdery rock debris, which, fortunately,
didn't happen). This covering is now called the regolith or also the lunar "soil". Regolith is produced by continual, repeated comminution of rock and rock fragments by multiple impact bombardments over millions of years. At full Moon, the dark areas (maria, now known to be mainly basaltic lavas that filled huge impact basins) have albedos of 5% to 8%, while brighter
areas, generally coincident with elevated terrain in the highlands (now proved to be feldspar-rich primitive lunar crust) range between 9% and 12%; these average out to about 7% for the full side facing Earth. These albedo differences are evident in this full Moon photograph, taken from the Lick Observatory in California (M. stands for Mare in the labels; A refers to Apollo Landing Sites
[in red]; and major craters show in yellow):
In the above photo, the two most obvious major features are the dark patches (maria, or basin lavas), with a smaller number of major craters, and the lighter-toned areas (terrae, or highlands), with numerous larger craters. Surprisingly, good maps of these major features are seldom found in texts or on the Web. Here is one that shows many of the best-known craters; note that the maria are labeled with their English translations of the original Latin used in the above full Moon image (those labels starting with M.):
The above Lick Observatory rendition is striking in its sharpness, but in achieving the tonal balance that highlights the craters, the ray deposits from Tycho, Copernicus, and others are strongly suppressed. Tycho (85 km rim-to-rim diameter) whose very long ejecta rays are visible to the naked eye from Earth, is very conspicuous in this next full Moon image in which the lighting and processing are optimized to display these ejecta deposits. Again, Tycho is near the bottom, with a dark rim, and at the locus of the converging light rays; this
implies that it formed recently enough (in the last billion years) so that the ray
material has not been obliterated.
19-3: How
many of the Apollo landings were in the maria; in the highlands? ANSWER
19-4: What
would have happened to the Apollo program if the lunar surface indeed had a
thick cover of powdery dust? ANSWER
Always a spectacular event, which happens rather frequently (a few years to a decade or so), is a lunar eclipse. In some parts of the world a total eclipse may occur; elsewhere the eclipse is partial. A lunar eclipse takes place when the Earth is on a line between the Sun and the Moon, blocking out much of the sunlight that had been illuminating the lunar surface (a solar eclipse results when the Moon moves across the line between the Sun and the Earth). Here is what the writer (NMS) observed in Bloomsburg, PA on the (uncommon) clear night of November 8, 2003 in which a near total eclipse was visible over much of the United States. The Moon can change color during different seasons. The best example is the "Harvest Moon", particularly associated with the Fall season. Then, the Moon has a distinctive orange tint. This modification from the normal yellow-white is largely the result of dust in the atmosphere scattering shorter wavelengths, owing to increased plowing as crops are harvested and fields cleared. (The appellation "Blue Moon" refers not to color but to the occasional situation when there are two full Moons in one calendar month.)
By selecting certain major features on the Moon's front face as reference points, one notes that the same face of the lunar sphere is seen during the entire period of its revolution around the Earth. This curiosity is the result of the Moon's sidereal revolution period of 27.3 days. The sidereal period is measured relative to a chosen star or star group. The Moon's rotation is almost exactly the same - 27.3... - as its sidereal revolution. In effect, a center point on the lunar frontside is seemingly locked onto the Earth, i.e., an imaginary line perpendicular to the lunar surface at that point would extend towards the Earth's center at all times during the Moon's orbital transit. This notable condition describes a state of captured or synchronous rotation. Because of this the Moon always presents approximately the same face to Earth observers. The synodic lunar month is 29.5 days. This increase in time by approximately 2.2 days results from the combined relative motions of both Earth and Moon forward in orbit around the Sun during which the illumination angle with respect to the Sun will be shifted; the Moon thus requires extra time for the first sliver of reflected light at New Moon to be seen from a reference point on Earth to compensate for the angular advance of the Earth-Moon system since the previous lunar month began.
19-5: What is the difference between rotation and revolution of a planetary body? ANSWER
19-6: Most people live in the Northern Hemisphere of Earth. It is a little known fact to many such individuals that the Moon is "upside-down" in the Southern Hemisphere, with Tycho near the top and Mare Imbrium towards the bottom. Can you explain why? ANSWER
Although the Moon is nearly a sphere, with one side always facing Earth, inspection of photographs taken on different dates discloses an apparent shift of major landmarks relative to reference point at the lunar limbs. Mapping demonstrates that almost 60% of the total surface (front and back) of the lunar sphere has been seen from Earth through telescopes. Because this seems to be a "wobbling" or "rocking back and forth", the term libration (from a Latin word for "balance") is applied to the phenomenon. Librations result in part from apparent (optical) displacements that bring more of both equatorial and polar regions into view. The motions involve variations in angular velocity of the Moon's revolution in elliptical orbit, tilt of its rotational axis relative to its orbital plane (inclined at 5°09' to the ecliptic), and parallax effects for different observation points on Earth. There is also a real libration owing to varying gravitational pull by Earth as the Earth-Moon distance varies and by the Moon's departure from perfect sphericity.
Like the Earth, that departure is due in large part to equatorial bulging owing to a once faster lunar rotation. In the early Earth-Moon history, the Moon was much closer to Earth [perhaps almost to the Roche Limit - the closest distance two large bodies can have stable orbits before the smaller one breaks apart from gravitational force - which for the Moon is ~2.9 x the Earth's radius or about 18000 km] but had then both a different rotation and revolution than today. Over time, tidal interaction between Earth and its satellite, which results in transfering angular momentum to the Moon, has caused 1) both the latter and Earth to slow down their rotations, and 2) the Moon to recede farther into space, expanding its orbit. However, on the Earth side (nearside or frontside; by agreement the term "backside" was declared "anathema", farside being preferred), there is an added component to the rotational bulging which occurs in the highlands (the farside surface is mostly highlands and is the manifestation of an early thicker crust). This so-called Lunar Bulge was described by G.A. Mills in 1968 as shown in this smoothed-contour illustration (the lines are in kilometer above and below a mean frontside elevation).
This bulge is not due to just higher crustal topography alone (note that the topographic relief is greater than 4 kilometers - almost 3 miles) that is brought out by the elevations in the flat lower maria. It appears to come from a thickening of the subcrust, with an increased density. Current interpretation holds it to have formed early in lunar history, when the Moon was much closer to Earth, as imposed by Earth's gravitational attraction that caused plastic rock to move towards the parent body. In time, the bulge was "frozen in".
Other remote sensing methods were applied to the Moon prior to the Space Age. Even before the pre-Apollo and Apollo programs, scientists examined the Moon in the multispectral mode (see a
recent example on page 19-6b). The simplest approach was to use photofilters
that pass limited spectral ranges. Thus, they examined the Moon in the UV, blue,
green, red, IR, and other spectral intervals. This technique brought out variations
in color shades that appeared to distinguish different surface units. Some maria
tended to have stronger blue components, while others were more reddish. Differences
between maria and highlands were accentuated. These subtle variations related
to the influence of elements, such as calcium, iron, and titanium, on the behavior
of reflected light at different wavelengths. Some investigators made more exacting
measurements by passing the light through a spectrometer, which provided semiquantitative
estimates of variations in percentages of several of the common elements. Higher
values for iron, magnesium, and calcium supported the surmise that low albedo lunar rocks
were most probably of basic igneous composition (silica-low, but Fe/Mg-rich).
Astronauts orbiting the Moon
described its color as dominantly medium to dark grays with tan overtones. We
have obtained many pictures of the lunar surface, starting with unmanned pre-Apollo
missions, then through numerous Apollo shots, and, afterwards, in a series of
images sent back by the Galileo spacecraft as it passed Earth enroute for its
rendezvous with Jupiter, and then by the Clementine orbiter (page 19-6b). This next photo shows a typical surface of the Moon as would be seen by astronauts orbiting above. The most obvious feature is indeed the most obvious component of the lunar landscape - impact craters of various sizes:
Large craters show three distinctive features attributable to impact (discussed in more detail in Section 18): 1) A raised rim; 2) Slumped walls; 3) A central peak.
First from orbiting spacecraft and then from the astronaut, humans got their first look at the lunar farside (never visible from Earth because the Moon is locked into a synchronous orbit). A typical image of the farside Full Moon, taken from the Apollo 12 Command Module as it orbited behind the earthside, shows much of the backside and part of the eastern limb seeable from Earth. Visible maria include Smythii, Marginis, and Crisium (near circular). This photo was taken when the Moon was largely dark as seen from Earth, but was between Earth and Sun
19-7: Considering the upper diagonal half of the above full Moon image to be typical of the farside, how does it differ from the full Moon view seen from Earth? ANSWER
The best known crater on the far side, imaged by the Russian Lunik and Zond spacecraft, and shown here in a higher resolution photo taken by an Apollo 9 astronaut, is Tsiolkovsky (named for a Russian lunar astronomer) with its filling by dark lavas.
As early as
1946, radar had provided the first ever images of the full Moon and selected regions.
Sent from Earth transmitters, signals returned as polarized backscatter, in
which we measure time delays and Doppler frequency shifts. Here are three mosaics
in Mercator (top), Lambert Conformal, and Polar Stereographic projections made
from reflections of 3.8 cm pulses sent by the Haystack Observatory at the Massachusetts
Institute of Technology: 
Just as was seen during our review of terrestrial remote sensing, the appearance of surface features can be quite different when optical and radar imaging are carried out. That is evident in this pair of images (visible left; radar right):
As decades past, ever better radar images were obtained from Earth. This view of the lunar south polar region, made with radar at Arecibo, confirms the statement:
Infrared scanners, operating through telescopes, can acquire thermal emission images of the Moon's surface. The ideal time to sense the full Moon is during a total lunar eclipse:
Under this condition, bright areas called "hotspots" appear, many of which, correlate with large lunar craters. These craters often contain dark lavas that absorb solar radiation (for more on this black body effect, see page 9-1) and re-emit it at higher radiant temperatures. During such times, observers also look for "lunar transients" (localized short-lived bright, often reddish glows visible to the eye) that some believe are evidence of volcanic activity and other thermal phenomena. Some people claim to see these volcanic events, even when the Moon is normally illuminated, particularly in the shadowed areas (dark phases).
19-8: Think of another possible explanation for these glowing transient phenomena. ANSWER
Some lunar scientists have been fascinated (almost obsessed) by lunar transients. The most frequent transient "hot spot" is the crater, Aristarchus, the brightest such feature on the Moon. Here is a Lunar Orbiter view of Aristrarchus:
Reports of events involving short-lived light emission at Aristarchus go back to the 6th Century. This plot of the days in the 29-day lunar month in which observations were recorded shows a provocative non-uniform distribution:
The above eclipse photo was taken more than 50 years ago from a ground telescope. In September 1999, a NASA spacecraft called MSX was in position around the Earth to image a total lunar eclipse using an IR channel. Here is the result; compare the hotspots here to those in the telescope image above.
The large bright spot is Tycho, which appears to be the hottest spot on the Moon's frontside.
Moon watchers, especially those in the amateur ranks, have been conditioned to look for transients whenever they are observing. (Much credit for this is due to Mrs. Wini Cameron, a friend of the writer's, who spent most of her professional life at NASA Goddard in promoting the validity of transients). Among this category of observations is the a controversial sighting, made by an amateur, Dr. Leon Stuart, that he also photographed through his home telescope. A strong visual flash appeared to him on November 15, 1953 for just a minute or so near the lunar terminator; his photo shows this as a bright dot.
At the time he interpreted this spot to be a glow from energy released when a meteorite struck the Moon's surface. Although believable, there were doubters, and the idea remained in dispute until 2002. Interpreters of the imagery telemetered back by the spacecraft Clementine (see page 20-6b) found an image that contained the estimated point of this possible impact. Here it is:
This has all the "earmarks" of a fresh crater. It is made visible by the more brightly reflecting material tossed out by the impact. From the crater size (1.5 km diameter), an incoming bolide 20 m in diameter upon collision released about 500 kilotons of energy as it dug out this now confirmed impact structure that is probably the most common type of landform on the Moon.
The detectability of a lunar impact as it happened was confirmed by the deliberate crashing of the European lunar explorer SMART-1 onto the surface. Telescopes on Earth observed a bright flash that was photographed. This bright spot greatly diminished afterwards. This is one such photo of the event, which took place on August 3, 2006:
This map shows the areas of the Moon at which a number of observations of transients make them "leaders" in the "hot spot" category:
Because most lunar observers have never seen a transient, this topic is still treated with skepticism. If transients or hot spots exist, the three most likely explanations are: 1) the light released comes from an impact, 2) there are gases emitted that are excited by particle bombardment, making them glow (usually red), and 3) some type of electrostatic discharge is involved.
This page closes with an Internet reference, to a NASA website that provides a synopsis of the Moon: Moon overview
Primary Author: Nicholas M.
Short, Sr.