Eccentric

An eccentric orbit is one formed about a centre, which itself is revolving about another centre. Ptolemy first employed the term as descriptive of the orbits of the planets about the Sun, viewed from the Earth as the central point of observation. His supposition was that the orbit was a circle, but that the Sun was not in the center of the orbit. In fact he considered it to be an imaginary circle representing an imaginary orbit, since it had not been discovered that the planets revolved round the Sun. The term is now applied to an elliptical orbit. The eccentricity of an elliptical orbit is defined astronomically as its degree of departure from a circle. It is expressed by the ratio of the major to the minor axis. The orbit of Venus has the least, and that of Mercury the greatest eccentricity of the planets in the solar system.