What shape is the moon? 


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Samuel Chiltern Profile
Samuel Chiltern answered
The Moon has a roughly oval shape that scientists refer to as an oblate spheroid, but astronomers haven't yet been able to agree why the Moon has an eccentric shape!

There are several theories, though, with the following being the most popular:

  • The Moon might have been locked into an oval orbit during its early formation, when it was much softer and more easily pulled out of shape
  • The Moon might have been hit by another moon orbiting Earth, and this could have knocked it out of shape
  • The Earth's own gravity might have pulled the Moon out of shape, again when it was still young, and before its surface had hardened 

The Lunar Phase

Don't confuse the actual shape of the Moon with its lunar phase - which is the current position of the Moon relative to the Earth and the Sun. 

The lunar phase explains why the Moon appears to change shape in the night sky, although of course in reality it does no such thing.

The sun is always illuminating half of the Moon at any point during the lunar phase, and so the only thing that changes is our viewpoint as it rotates around the Earth.

When the moon is between the sun and our planet, we can't see the illuminated side at all, because it is facing the Sun.

When the Moon is exactly on the opposite side from the Sun, we can see all of the half that is illuminated. 

At any point in between these two stages, we see an increasing (waxing), then decreasing (waning) amount of the surface being illuminated.

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