Page:A Voyage in Space (1913).djvu/37

Rh —Kepler's Laws. The first is that the planets move round the Sun in ellipses. You can easily draw an ellipse for yourself with two drawing-pins and a loop of string. Fix the pins in a

and put the loop of string round them, keeping it stretched into a triangle with a pencil point. You can still move the pencil about and it will draw you an ellipse. The drawing-pins are called the foci of the ellipse, and when a planet moves in an ellipse the Sun is at one of the foci. Once when the late