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The Moon is the only natural satellite of Earth:
orbit: 384,400 km from Earth
diameter: 3476 km
mass: 7.35e22 kg
The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.
Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.
The
Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been
visited by humans. The first landing was on July 20, 1969; the last
was in December 1972. The Moon is also the only body from which
samples have been returned to Earth. In the summer of 1994, the Moon
was very extensively mapped by the little spacecraft Clementine. Lunar
Prospector is now in orbit around the
Moon.
The gravitational forces between the
Earth and the Moon cause some interesting effects. The most obvious
is the tides.
The Moon's gravitational attraction is stronger on the side of the
Earth nearest to the Moon and weaker on the opposite side. Since the
Earth, and particularly the oceans, is not perfectly rigid it is
stretched out along the line toward the Moon. From our perspective on
the Earth's surface we see two small bulges, one in the direction of
the Moon and one directly opposite. The effect is much stronger in
the ocean water than in the solid crust so the water bulges are
higher. And because the Earth rotates much faster than the Moon moves
in its orbit, the bulges move around the Earth about once a day
giving two high tides per day.
The Moon rotates
synchronously, i.e. it is locked in phase with its orbit so that the
same side is always facing toward the Earth.
Actually, the Moon appears to wobble a bit (due to its
slightly non-circular orbit) so that a few degrees of the far side
can be seen from time to time, but the majority of the far side
(left) was completely unknown until the Soviet spacecraft
Luna
3 photographed it in 1959. (Note: there is
no "dark side" of the Moon; all parts of the Moon get sunlight half
the time. Some uses of the term "dark side" in the past may have
referred to the far side as "dark" in the sense of "unknown" (eg
"darkest Africa; but even that meaning is no longer valid
today!)
The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well. The cost of future lunar exploration just got a lot cheaper!
The Moon's crust averages 68 km thick. Below the crust is a mantle and probably a small core (about 2% by mass; about 300 km radius). Unlike the Earth's mantle, however, the Moon's is only partially molten.
There are two primary types of terrain on the Moon: the
heavily cratered and very old
highlands and
the relatively smooth and younger maria. The maria (which comprise
about 16% of the Moon's surface) are huge impact craters that were
later flooded by molten lava. Most of the surface is covered with
regolith, a mixture of fine dust
and rocky debris produced by meteor impacts. For some unknown reason,
the maria are concentrated on the near side.
Most of the craters on the near side
are named for famous figures in the history of science such as
Tycho,
Copernicus, and Ptolemaeus. Features on the far have more modern references such as
Apollo, Gagarin and Korolev (with a distinctly Russian bias since the
first images were obtained by Luna
3). 
In addition to the
familiar features on the near side, the Moon also has the huge
craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km
deep making it the the largest impact basin in the solar system and
Orientale on the
western limb (as seen from Earth; in the center of the image at left)
which is a splendid example of a multi-ring crater.
A total of 382 kg of rock samples were returned to the Earth
by the Apollo and Luna programs. These provide most of our detailed knowledge of
the Moon. They are particularly valuable in that they can be dated.
Even today, 20 years after the last Moon landing, scientists still
study these precious samples.
Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is lucky because earth rocks are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth.
New and detailed information
from the Moon rocks led to the impact theory: that the Earth
collided with a very large object (as big as Mars or more) and that
the Moon formed from the ejected material. There are still details to
be worked out, but the impact theory is now widely accepted.
The Moon has no global magnetic field.
With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar wind. Over its 4 billion year lifetime many hydrogen ions from the solar wind have become embedded in the Moon's regolith. This lunar hydrogen may also be of use someday as rocket fuel.
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Earth ... Moon ...
Mars ... 
