Back to  Why is the Moon a Sphere and what is it made of?

Room 7

Gravity pulls everything toward the centre. While the Moon was forming, when it was still small, its gravity was low and it was probably very irregular in shape,

Asteroids are small and often lumpy. GASPRA is the name of an asteroid that was photographed several years ago and it looks like a potato. Since it is small, it's gravity is not very strong.

As the Moon got bigger by collecting rocks that hit it, the force of gravity got larger and the rocks would roll and bounce toward the low spots before sticking. This way, the low spots got filled in and the Moon became smoother.

Do you know what shape has no low or high points? A sphere or ball. So the force of gravity tends to make the Moon round.

Imagine what would happen today if part of the Moon stuck out like the corner of a cube. It would be farther away from the centre. If any pieces broke off for any reason, they would tumble down away from the high spot. So the high spots would get worn down until the Moon was round. That's called erosion.

If it weren't for large asteroids hitting the Moon, the Moon would be worn down to a nearly perfect sphere by now.

Here are some pictures of lumpy Gaspra as well as one of Mars' lumpy moons and another asteroid. You can see the Moon for yourself by looking in the sky at night. It's not always visible though becuase it moves around the Earth and sometimes is overhead during the day when it's too bright to see.

Answer #% % filename A118.DOC page% % page 1 of% % numpages 1 % % date \@ "MMMM d, yyyy"March 20, 1996

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