| Moon, name given to the
natural satellite of Earth, and sometimes applied to the satellites
of the other planets in the solar system. The diameter of Earth’s
Moon is about 3,480 km (about 2,160 mi), or about one-fourth that
of Earth, and the Moon’s volume is about one-fiftieth that of Earth.
The mass of Earth is 81 times greater than the mass of the Moon. Thus
the average density of the Moon is only three-fifths, and the gravitational
pull at the lunar surface only one-sixth, that of Earth. The Moon
has no liquid water and essentially no atmosphere, so no weather exists
to change its surface; yet it is not totally inert.
The Moon moves about Earth at an
average distance of 384,403 km (238,857 mi), and at an average speed
of 3,700 km/h (2,300 mph) .
It completes one revolution in
an elliptical orbit about Earth in 27 days 7 hours 43 minutes 11.5
seconds with reference to the stars. For the Moon to go from one
phase to the next similar phase, or one lunar month, requires 29
days 12 hours 44 minutes 2.8 seconds. The Moon rotates once on its
axis in about the same period of time that elapses for its sidereal
period of revolution, accounting for the fact that virtually the
same portion of the Moon is always turned toward the Earth. Although
the Moon appears bright to the eye, it reflects into space only
7 percent of the light that falls on it. The reflectivity, or albedo,
of 0.07 is similar to that of coal dust.
At any one time, an observer can
see only 50 percent of the Moon’s entire surface. However, an additional
9 percent can be seen from time to time around the apparent edge
because of the relative motion called libration. This is because
of the slightly different angles of view from Earth, due to different
relative positions of the Moon along its inclined elliptical orbit.
The Moon shows progressively different phases as it moves along
its orbit around Earth. Half the Moon is always in sunlight, just
as half Earth has day while the other half has night. The phases
of the Moon depend on how much of the sunlit half can be seen at
any one time. In the phase called the new moon, the face is completely
in shadow. About a week later, the Moon is in first quarter, resembling
a luminous half-circle; another week later, the full moon shows
its fully lighted surface; a week afterward, in its last quarter,
the Moon appears as a half-circle again. The entire cycle is repeated
each lunar month. The Moon is full when it is farther away from
the Sun than Earth; it is new when it is closer. When it is more
than half illuminated, it is said to be in gibbous phase. The Moon
is said to be waning when it progresses from full to new, and to
be waxing as it proceeds again to full. Temperatures on its surface
are extreme, ranging from a maximum of 127°C (261°F) at lunar noon
to a minimum of -173°C (-279°F) just before lunar dawn.
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