The Moon is the only natural satellite of Earth: orbit: 384,400 km from Earth
diameter: 3476 km mass: 7.35e22 kg Called Luna by the Romans, Selene and Artemis
by the Greeks, and many other names in other mythologies. 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 (do
you remember where you were?); 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
and again in 1999 by Lunar Prospector.
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. But the Earth is not completely fluid, either. The Earth's
rotation carries the Earth's bulges get slightly ahead of the point directly
beneath the Moon. This means that the force between the Earth and the Moon is
not exactly along the line between their centers producing a torque on the Earth
and an accelerating force on the Moon.
This causes a net transfer of rotational energy from the Earth to the Moon,
slowing down the Earth's rotation by about 1.5 milliseconds/century and raising
the Moon into a higher orbit by about 3.8 centimeters per year. (The opposite
effect happens to satellites with unusual orbits such as Phobos and Triton). The
asymmetric nature of this gravitational interaction is also responsible for the
fact that 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.
Just as the Earth's rotation is now being slowed by the Moon's influence so
in the distant past the Moon's rotation was slowed by the action of the Earth,
but in that case the effect was much stronger. When the Moon's rotation rate was
slowed to match its orbital period (such that the bulge always faced toward the
Earth) there was no longer an off-center torque on the Moon and a stable
situation was achieved. The same thing has happened to most of the other
satellites in the solar system. Eventually, the Earth's rotation will be slowed
to match the Moon's period, too, as is the case with Pluto and Charon.
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 and varies from essentially 0 under Mare
Crisium to 107 km north of the crater Korolev on the lunar far side.
Below the crust is a mantle and probably a small core (roughly 340 km radius
and 2% of the Moon's mass). Unlike the Earth's mantle, however, the Moon's is
only partially molten. Curiously, the Moon's center of mass is offset from its
geometric center by about 2 km in the direction toward the Earth. Also, the
crust is thinner on the near side. 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.
