Gravity, Atmosphere, and MotionThe good news about the Moon is that 700 pound circus legend Baby Ruth Pontico would weigh in at a svelt and self-affirming 117 pounds, its gravity being only about one-sixth that of the Earth. But of course, you know the bad news - although she may have indeed shed some "weight", Ruth's real problem is perhaps better considered in terms of "mass". If Ms. Pontico should choose to visit the Moon one day, better that it should be for some of the other fascinating and beautiful qualities introduced in the next few pages.
Although no atmosphere means no water vapor, a large patch of water ice discovered near the Moon's south pole has recently changed the way we think of our barren neighbor. Preserved in a zone of permanent shadow below lunar "sea level" called a cold trap, the water probably arrived as an ill-fated comet long ago. Its availability as a resource to Moon-based colonies is an exciting possibility. With no gaseous medium to moderate the effects of solar radiation, the temperature swings dramatically from day to night. These diurnal extremes of from -184°C (-300°F) to 101°C (214°F) throughout most of the Moon's regions is typical, while the poles tend to remain a relatively constant -96°C (-140°F). The influence of Earth's gravitational force on the Moon is strong. It locks the Moon into a 27.32 day captive orbit, and grabs onto a mascon, or area of densely concentrated mass, and prevents it from rotating. This synchronous rotation means that the Moon spins on its axis at exactly the same rate at which it orbits the Earth, and we always see the same face of the Moon. The near hemisphere was all we knew of the Moon until camera-bearing spacecraft first peeked around the far side in the early 1960's. A phenomenon called libration actually allows an earthly observer to view 59% of the lunar surface at one time or another. There are two kinds of libration - optical and physical. Optical libration is simply the result of the parallax inherent in viewing the Moon from one extreme point in the Earth's rotation and then the other. The point of view of an observer on the equator, for example, will shift 12,756km in space after 12 hours of rotation. The observer's eyes are therefore sweeping a slightly different area of lunar surface. Physical libration, on the other hand, is the result of the Moon itself shifting its orientation in space. Earth's gravity is constantly tugging and pulling on the Moon, and because density varies across its surface, the Moon actually wobbles as it orbits. As it wobbles, the face it reveals to us changes.
The Truth About Tides
The influence of the Moon's gravity upon the Earth is the primary source of our ocean tides. Newton had found that the attractive force of an object is inversely proportional to the square of the distance between the objects. This is known as the inverse square law. The first to apply this law to the question of tides was the Marquis de Leplace (1749-1827). Just as the attractive force imposed on a series of ball bearings radiating from a large and powerful magnet would decrease as distance increased, so too different points along a line from the Moon, passing straight through the Earth, would incur a decreasing attraction with distance. This means that some parts of the Earth are being pulled harder than others. The result is a pulling of the Earth "out of round" as it spins through that imaginary line of gravitational force radiating from the Moon. Laplace realized that the effect of lunar gravity on the side of the Earth facing the Moon had to be greater than its effect on the center of the Earth (which is a greater distance from the Moon), and weakest of all on the side facing away from the Moon. Thus our poor little planet is being continuously deformed and we experience two simultaneous high tides, one on the lunar-facing side as a result of the Earth swelling toward the Moon; and one on the opposite side as the Earth's crust, now closer to the Moon than the ocean above it, is drawn to the Moon while the waters above are allowed to bulge away. Two low tides are naturally experienced at the points 90° to each point of high tide.
The Lunar Surface
As you get closer you begin to see more detail. Major impact craters, over 30,000 on the near side alone, gradually come into view. These range in size anywhere from a few hundred feet to over 200 miles across. Patterns of rays emanating from the great Tycho and Copernicus craters are actually streaks of ejecta sent flying by the force of powerful impacts. Soon mountain ranges, formed not by the collisions of lithospheric plates as on Earth, but by the tremendous thrusting forces of giant meteor impacts, become visible around some of the maria. The Apennines, a 600 km range with peaks as high as 5,000 meters forming the eastern rim of Mare Imbrium, are now clearly visible. Eventually, vein-like wrinkle-ridges and raised domes, convex features which are formed as the surface gives way to internal pressures, can be seen in and around the maria. Faults which resemble long, winding trenches known as rilles, or clefts, also scar the surface. Billions of years of meteoritic bombardment have left the Moon with a fine, granular soil called the regolith. The crew of Apollo XV described it as being "like soft, powder snow". Analyses of lunar rocks reveals no water bound into their molecular structure. The Moon's far side is more vulnerable to meteor, asteroid, and comet impacts because it doesn't have the Earth on its side; literally. An object heading in a straight line for the center of the Moon's near side would have to get through the Earth first. Much like a seaside village facing a hurricane, the far hemisphere of the Moon catches more grief than its partially protected side. It is therefore more rugged and cratered, with virtually none of the contrasting maria so abundant on the Earth-facing side.
The Phases of the Moon
Once a month the Earth is sandwiched between the Sun and the Moon. With the light source, observation platform, and object of interest lined up this way, we see the entire illuminated portion of the lunar disk - a full moon. Halfway through the month the Moon has reached a point in its orbit around the Earth where it is now sandwiched between the Earth and Sun. The illuminated portion of the Moon's surface is now facing away from the Earth, and we see a new moon.
Eclipse
When the Earth is between the Sun and Moon, and in such perfect alignment that it casts its shadow across the Moon, a lunar eclipse is in progress. To put it another way, when the Moon just happens to get in the path of the Earth's umbra, or projected cone of shadow, it will be eclipsed. The Moon often appears reddish during a lunar eclipse. A small amount of light filters through the Earth's atmosphere, losing its blue hues for its trouble. Variations in shading are created as scattered cloud layers absorb some of the light. When the Moon is between the Sun and the Earth and casts its shadow on the Earth, a solar eclipse takes place. Due to an uncanny coincidence, the Moon and Sun appear in our sky to be exactly the same size. Sure, the Moon is 400 times smaller than the Sun, but it also just happens to be exactly 400 times closer. Strange, but true. Thus it is possible for the lunar disk to precisely cover the Sun when the alignment is just right. During a solar eclipse the retreating solar crescent and eerie, other-worldly shadows are followed by a rare opportunity to observe phenomena such as Baily Grains, beads of light that peek through the lunar mountain gaps just before the moment of totality. While total, the Sun's chromosphere and outer corona, obscured at all other times by the intensity of its own light, are visible for a few minutes. Wait a minute. This all sounds a lot like the Moon phase story. What gives? Although the Sun, Earth, and Moon would appear to be perfectly aligned during an average full moon if viewed from above, an edge-on view would reveal that the orbital planes of each body are a little different. They line up horizontally, you might say, but not vertically. All three bodies must be passing through the ecliptic plane at the same time in order to achieve eclipse, and that is predictably rare.
Its OriginNo single answer to the question of the Moon's origin has yet been universally accepted. Most scientists agree that it formed very early in the history of our solar system, about 4.5 billion years ago. There are currently four popular hypotheses that attempt to explain the Moon's formation. In the coaccretion hypothesis, the Earth and Moon are a double planet, condensing simultaneously out of a single source of gas and dust in one corner of the nebula from which our solar system formed. The fissure hypothesis has the primeval Earth reaching such a furious speed in its rotation that it actually spins off a portion of its mass, which then condenses into our Moon. The capture hypothesis suggests that the Moon formed elsewhere in the infant solar system, wandered too close to the Earth one day, and assumed an orbit about the Earth's superior mass. One theory has moved to the fore in recent years, however, primarily because it better fits the accumulating scientific data. In the giant impact hypothesis, a body approximately the size of Mars collides with the Earth, vaporizing the upper mantles of both objects almost entirely. Gravity draws the metallic, mostly iron core of the smaller object through the lower mantle and into the core of the larger object. The volatile elements, those which boil at lower temperatures such as water, are vaporized in the heat of the collision. The debris that remains is rich in the refractory elements, or those that boil at higher temperatures, such as titanium. It is this material which coalesces to form the Moon.
ConclusionsWhy continue to explore the Moon? Because of its weaker gravity, heavy elements were more evenly distributed throughout the Moon's total mass during formation, and can be mined on or close to its surface.Weak gravity also means a low escape velocity, making it an attractive base of operations for solar system explorations. But the most convincing argument is certainly the pursuit of knowledge. As we attempt to better understand the universe which we inhabit, our home planet leaves us wanting for important types of evidence. The oldest rock sample ever found on Earth is about 4.1 billion years old - not bad, except that is an extremely rare and lucky find. Few can be dated much farther back than 3 billion years. That leaves a full third of our solar system's history unrepresented by Earthly samples. The Moon, on the other hand, is without the liquid water, atmosphere, and volcanism that lead to surface erosion and plate tectonics. Its rocks and soil remain virtually unchanged, and so the answers to many important questions lie scattered across its still surface, waiting patiently to be discovered. Bruce R. Mattson is the Science and Technology Specialist at the McAuliffe/Challenger Center For more on the Moon...If you can pick Wilma Flintstone out of a crowd, you can easily identify many important lunar features. Try it. ESA's SMART-1 will test miniaturisation technology while exploring the Moon from orbit. A Java animation describing the phases of the Moon. Browse some photos of the U.S. and Russian Apollo/Soyuz vehicles on display at the National Air & Space Museum. The National Space Science Data Center has an excellent Moon Fact Sheet, including Moon/Earth comparison data. Apollo program enthusiasts will enjoy Apollo To The Moon, an on-line exhibit at the National Air & Space Museum. A detailed Apollo program chronology from NASA.
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The Moon is significantly less dense than the Earth. Although
it is about a quarter the diameter of the Earth, the Moon
contains barely over 1% of the mass that the Earth contains.
Because gravity is directly proportional to an object's mass, the
Moon has a relatively weak gravitational field when compared to
denser bodies with similar volume. One side effect of this weak
gravity is an inability to retain an Atmosphere.
Any gases or water that might have been present during its
formation would have quickly dissipated into space. 
The same force that produces violent volcanism on Jupiter's
moon Io causes the Earth's oceans to rise and fall. This tidal
force is the product of the gravitational influence
which one body imposes on another.
To best visualize the terrain, imagine that you are beginning
a journey to the Moon. Even from an Earthbound vantage point the maria
and terrae would be easily discernible. They are the
contrasting areas which create the man-in-the-moon effect. Maria
are the younger of the two features, and consist of low-lying
plains of volcanic basalt partially filling older impact basins.
They reflect somewhat less than the 7% lunar average of sunlight
received, a characteristic known as albedo, and
so appear darker than surrounding features. Terrae,
also known as highlands, are elevated, often
mountainous and heavily cratered regions covering about 84% of
the Moon's surface.
The ratio of illuminated to non-illuminated portions of the
lunar disk, as seen from Earth at any time during its monthly
cycle, is known as its phase. Naturally, half of
the Moon's surface is always illuminated by the Sun (except
during a lunar eclipse); the change of appearance occurs because
the Moon, the Sun, and the Earth are constantly changing position
relative to one another. 
When one body passes into the shadow of another body, an eclipse
occurs. An eclipse may be partial, if the interloper sweeps only
a portion of the eclipsed body; or total, if the
disk of the interloper crosses squarely in front of the eclipsed
body, masking its entire face.