For centuries astronomers had felt that planetary orbits involved some form of circular motion. This belief, however, had not proved true in actual observation, and scientists were led to extremely complex diagrams and equations to explain the discrepancies. Kepler, after years of calculation, primarily with regard to the planet Mars, arrived at the conclusion that this planet’s orbit was not circular but a geometric figure called an ellipse.

Most of the planets travel in orbits that are nearly circular, the earth’s orbit being almost a perfect circle. A few planets, however, have elliptical paths that are quite eccentric, that is, they are flatter or less round. Pluto and Mercury are the most eccentric of the major planets, but some comets, such as the famous Halley’s Comet, have extremely eccentric orbits.