The Earth with its moon, as seen from space.
Click on image for full size
NASA.
The Moon's Orbit and Rotation
The Earth's Moon is the fifth largest in the whole solar system, and
is bigger than the planet Pluto. The Moon has a nearly circular orbit
(
e=0.05) which is tilted about 5° to the
plane of the Earth's orbit. Its average distance from the Earth is
384,400 km. The combination of the Moon's size and its distance from
the Earth causes the Moon to appear the same size in the sky as the
Sun, which is one reason we can have total solar eclipses.
The Moon's orbital period is 27.322 days. Because of this motion, the
Moon appears to move about 13° against the stars each day, or about
half of a degree per hour. If you watch the Moon over the course of
several hours one night, you will notice that its position among the
stars will change by a few degrees. The changing position of the Moon
with respect to the Sun leads to lunar
phases.
Have you ever heard the term the 'far-side' of the Moon? Because of
the effect on the Moon of tidal
forces due to the Earth, the same side of the moon always faces
the Earth. The rotation period and the orbital period of the Moon are
the same. Therefore, Earth-bound observers can never see the
'far-side' of the Moon. Tidal forces cause many of the moons of our
solar system to have this type of orbit.
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