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.
It takes the Moon 27.322 days to go around the Earth once. Because of
this motion, the Moon appears to move about 13° against the stars
each day, or about one-half 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. In other words, it takes the Moon the same amount of time
to rotate around once as it does for the Moon to go around the
Earth once. 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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