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The Moon's Orbit and Rotation - Windows to the Universe

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The Earth with its moon, as seen from space.
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NASA.

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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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF