The Earth with its moon, as seen from space.
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The Moon's Orbit and Rotation
All the planets except Venus and Mercury have moons. The Earth's Moon
is the fifth largest in the whole solar system, and is bigger than the
planet Pluto. Earth's gravity pulls on the Moon and keeps it in
orbit. The Moon's orbit is almost a perfect circle, so the Moon is
about 384,400 km away all the time. Although the Sun is really much
bigger than the Moon, the Moon is much closer, so it appears to be the
same size as the Sun in our sky. A total eclipse occurs when the Moon
is in the right position to just cover up the Sun.
It takes the Moon about 27 days to go around the Earth once. If you
check on the Moon several times during one night, you will notice that
it moves relative to the stars around it. As the Moon goes around the
Earth, different portions of it are lit up by the Sun, causing lunar phases. It takes the Moon one month to go
through all its phases.
Have you ever heard the term the 'far-side' of the Moon? The Earth's
gravity produces tidal
forces on the Moon. This causes the same side of the moon to
always face the Earth. People living on the Earth can never see the
'far-side' of the Moon, unless they go there! Tidal forces cause many
of the moons of our solar system to always face their planets.
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