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The Question

(Submitted January 04, 1998)

I want to know why aren't the planets' orbits circular?

The Answer

Many ancient astronomers thought that the orbits of the planets should be circular, because it's simple and it seemed very natural to them.

But when Newton discovered the laws of motion and gravity, the natural answer turned out to be elliptical orbit. A circular orbit is allowed, but requires a very special set of conditions. A circular orbit occurs when the kinetic energy of an orbiting body is exactly equal to half of its negative potential energy, and its direction of travel is exactly perpendicular to the direction from the orbiter to the thing orbited. The only way to get an exact circular orbit is by carefully fiddling with the parameters, e.g. by firing rockets. Nature does not usually fiddle, so natural orbits will all be elliptical to some extent.

Best wishes,

David Palmer and Koji Mukai
for "Ask an Astrophysicist"

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