At one time, it was thought that the planets in our solar system revolve around the sun in a circular orbit. It was later discovered, however, that the orbits are not circular, but were actually very round elliptical shapes. (Recall the discussion of the eccentricity of an ellipse mentioned above.) The eccentricity of the orbit of the Earth around the sun is approximately 0.0167, a fairly small number. Pluto's orbit has the highest eccentricity of all the planets in our solar system at 0.2481. Still, this is not a very large value.
As a matter of fact, the sun acts as one of the foci in the ellipse. This phenomenon was first noted by Apollonius in the second century B.C. Kepler later studied this in a more rigorous fashion and developed the scientific view of planetary motion.
James A. Sellers