While flipping through my old high school calculus text the other week, checking something about ellipses for another course, I found this: The ratio /a) = e, called the eccentricity of the ellipse, varies from 0 to 1 and indicates the degree of departure from circularity. ... Most of the planets, including the earth, have orbits that are nearly circular...Pluto, however, has a fairly eccentric orbit, with e=0.25, as does Mercury...Icarus, an asteroid about 1 mile wide that revolves ar (chordiearchive.chordie.com)