ELLIPSE’

one of the conic sections. The curve generated by a point which moves so that its distance from a fixed point bears a constant ratio (le than unity) to its distance from a fixed straight line. Kepler discovered that the paths described by the planets in their revolutions round the sun are ellipses, the sun being placed in one of the foci. To describe an ellipse: At a given distance on the surface on which the ellipse is to be described fix two pins, A and B, and pa a looped string round them. Keep the string stretched by a pencil, c, and move the pencil round, keeping the string at the same tension, then the ellipse EGFH will be described. A and B are the foci, D the ‘centre, EF the major axis, GH the minor axis, and the fraction DA/DE the eccentricity of the ellipse. A line drawn from any point in the curve perpendicularly to the axis is an ordinate to the axis. Any straight line drawn through the centre and terminated both ways by the curve is called a diameter.