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Asked by modebayo to Stephen, Lauren, Joseph, Eoin, Colm on 12 Nov 2013. This question was also asked by tmcdonnell.
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Eoin O Colgain answered on 12 Nov 2013:
Don’t know if you’ve heard of button moon? It was a children’s program when I was younger. The moon was a button and indeed, from afar it does look like a circle.
However, planets are spherical (they are 3D objects). Since they all rotate, they tend to “pancake”, but only marginally. This means that the earth is not quite a sphere and is flatter at the poles than the equator.
So if there was no rotation, no day and night, they would be perfect spheres, or circles when viewed from a distance.
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colm bracken answered on 12 Nov 2013:
As Eoin said they appear circular because they are spheres (balls). The reason they take on ball shapes is because gravity pulls the material inward in all directions. If gravity is not too strong the perfect ball shape might not be achieved. This is why most asteroids have irregular shapes an sometimes look potato shaped.
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Stephen Scully answered on 14 Nov 2013:
It is shaped like a football. Everything wants to be pulled to the centre of the Earth, that is why we don’t float off. So over a really long time and the bits of the planet smooth out and like a ball, every part of the surface of the ball is the same distance from the centre of the ball. This is what the Earth tries to do too.
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