The Moon, Earth's nearest
celestial neighbor, is only 380,000 km away from us on average, so
close that it appears sufficiently large and bright to dominate our
nighttime sky. The Moon's mass is only 1/81 that of the Earth, and
the gravity at its surface is only 1/6 that of the Earth. Essentially
all of any atmosphere and any water that may once have been present would
long since have escaped into space. The Moon is about 1/4 the
diameter of the Earth - most solar system moons are much smaller fractions
of the sizes of their parents.
The Moon rotates on its axis at the
same rate as it revolves around the Earth, always keeping the same face in
our direction. The Earth's gravity has locked the Moon in this pattern,
first causing a bulge in the distribution of the lunar mass and then
interacting with the bulge to prevent the moon from rotating freely.
Even a small telescope reveals a
surface pockmarked with craters. The highlands are heavily
cratered. Other areas called maria (singular mare) are
relatively smooth. Besides maria and highlands other types of structures
visible on the Moon include mountain ranges and valleys. The
mountains are formed by debris, though, unlike mountains on Earth, which are
formed from plate tectonics. Lunar rilles are cracks that can extend
for hundreds of kilometers along the surface. Some are relatively straight
while others are sinuous. Raised ridges also occur. The craters
themselves come in all sizes, ranging from as much as 295 km across for
Bailly down to tiny fractions as a millimeter. Crater rims can be as
much as several kilometers high over the crater floors. Most existing
craters resulted from meteoritic impact, though some small fraction may have
come from volcanism. Though only a few craters may have resulted from
volcanism, there are many other signs of volcanic activity, including the
lava flows that filled the maria. The Maria Hills in Oceanus Procellarum,
for example, have many domes and rilles that apparently resulted from
repeated volcanism. More lavas may have flowed from such areas.
When the Moon is full, it is bright enough to cast
shadows or even to read by. But full moon is a bad time to try to observe
lunar surface structures, for any shadows we see on the Moon surface are
short. When the Moon is a crescent or even a half moon, however, the part of
the Moon facing us is covered with long shadows. The lunar features then
stand out in bold relief. Shadows are longest near the Terminator,
the line separating day from night.
The Moon makes an orbit of the Earth, as seen from far
away from the Earth-Moon system, in 271/3
days, the sideral revolution period of the Moon. But since the
Earth is moving in its orbit around the Sun, the phases repeat with a
different period. The Moon makes an orbit of the Earth in respect to the
position of the Sun in 291/2
days; the interval between successive new moon is the synodic revolution
period of the Moon. Because the same side of the moon always faces the
Earth, different regions face the Sun as the moon orbits. As a result the
terminator moves completely around the Moon with this 291/2-day
synodic period. Most locations on the Moon are thus in sunlight for about
15 days, during which they become very hot - 130
0C - and then in darkness for
about 15 days, during which their temperature drops to as low as
-110 0C. The cycle of phases
that we see from Earth also repeats with this 291/2-day
period - a synodic month.
The space age began on October 4, 1957, when the
U.S.S.R. launched its first Sputnik (the Russian word for "traveling
companion") into orbit. The shock of this event galvanized the American
space program, and within months American spacecraft were also in Earth
orbit. After a series of one-person and two-person spacecraft in orbit and
parallel robotic exploration of the Moon, the three-person Apollo
missions began. In Apollo 8, three astronauts circled the Moon on
Christmas Eve 1968 and returned to Earth. About six months later,
Apollo 11 brought humans to land on the moon for the first time. It went
into orbit around the moon after a three day journey from Earth, and a small
spacecraft called the Lunar Module separated from the larger
Command Module. On July 20, 1969 - a date from that from
the long-range standard of history may be the most significant of the
millennium - Neil Armstrong and Buzz Aldrin left Michael
Collins orbiting in the Command Module and landed on the Moon. Six
Apollo missions in all, ending with Apollo 17 in 1972, carried people
to the Moon.