Figure 2-8Ellipses (a) The construction of an ellipse: At all places along an ellipse, the sum of the distances to the two foci is a constant. An ellipse can be drawn with a pencil, a loop of string, and two thumbtacks, as shown. If the string is kept taut, the pencil traces out an ellipse. The two thumbtacks are located at the two foci of the ellipse. (b) A series of ellipses with different eccentricities (e). Eccentricities range between 0 (a circle) to just under 1.0 (almost a straight line). Note that all eight planets have eccentricities of less than 0.21. (c) Mercury has an especially eccentric orbit around the Sun.