Figure 3-14 Ellipses (a) To draw an ellipse, use two thumbtacks to secure the ends of a piece of string, then use a pencil to pull the string taut. If you move the pencil while keeping the string taut, the pencil traces out an ellipse. The thumbtacks are located at the two foci of the ellipse. The major axis is the greatest distance across the ellipse; the semimajor axis is half of this distance. (b) This shows a series of ellipses with the same major axis but different eccentricities. An ellipse can have any eccentricity from e = 0 (a circle) to just under e = 1 (virtually a straight line).