Terry Moffat of Rongotai College asks :-

Why do planets move in elliptical rather than circular orbits?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

Planets move in elliptical orbits for two reasons. One is the way gravity works. The other is due to a physical law about spinning masses. It is called the law of conservation of angular momentum.

The gravity pull of the sun on a planet varies inversely as the square of the distance from the sun. Double the distance and the gravity is one-quarter as strong. Three times further out it is one-ninth, and so on. So objects falling toward the sun speed up as they approach it. Objects moving away from the sun are slowed down a lot when they are near the sun but less so as they move further away.

You can see the law of conservation of angular momentum if you twill a weight on a string. Pull the string shorter and the twirling speeds up. This effect is used by ice-skaters and ballet dancers: spinning slower when their arms are extended; spinning faster when their arms are close to the body. The planet has a slower rotation around the sun when it is a long way out. As it falls closer the rotation speeds up. This causes the planet to get flung around the sun at its closest approach and directed outward again.

This effect was noted by Johannes Kepler early in the 17th Century. Kepler showed that the line joining a planet to the sun sweeps out equal areas in equal times: the planet moves faster when close to the sun than when further out. Angular momentum is conserved.

So an ellipse is the natural way that things move under gravity. Most of the big planets move in nearly circular orbits. A circle is just a special case of an ellipse. The moons of planets move around their planet in ellipses, though some of them are also affected by the gravity of the sun. When you throw a ball it moves on an arc that is a small part of an ellipse around the earth's centre, or would be if the air didn't slow it down.