With the help of Tycho Brahe's observations, Kepler discovered that the orbits of the planets can be described with a curve. By trial and error, he discovered that an ellipse with the sun could accurately describe the path of a planet about the sun. Ellipses (which does not look like the usual circle, but as an egg) is described mainly by the length of its two axes. A circle has the same diameter if we measure it across or up and down. However, an ellipse has diameters of different lengths. The longest may be termed the major axis, and the shortest one is the minor axis. The ratio of these two lengths determines the eccentricity (e) of the ellipse; it is a measure of how elliptical it is. Circles has E = 0, and very stretched out ellipses have
