Note 2 — BOOK II. THE DYNAMICAL PERIOD

An ellipse is one of the plane, sections of a cone. It is an oval curve, which may be drawn by fixing two pins in a sheet of paper at S and H, fastening a string, SPH, to the two pins, and stretching it with a pencil point at P, and moving the pencil point, while the string is kept taut, to trace the oval ellipse, APB. S and H are the foci. Kepler found the sun to be in one focus, say S. AB is the major axis. DE is the minor axis. C is the centre. The direction of AB is the line of apses. The ratio of CS to CA is the excentricity. The position of the planet at A is the perihelion (nearest to the sun). The position of the planet at B is the aphelion (farthest from the sun). The angle ASP is the anomaly when the planet is at P. CA or a line drawn from S to D is the mean distance of the planet from the sun.