Epicycles and Epicycle Orbits

Jon Ingram
Epicycles were a way of explaining the complicated motion of the planets in a system with the Earth, rather than the Sun, at the centre of the solar system. The motion of the planets seems to vary in speed, and can at some times even appear to move 'backwards'. One way to produce this motion is to have the planets moving on circles-within-circles, and even circles-within-circles-within-circles. The white blob shows a planet's motion as it would appear from an Earth at the centre. To see how the motion is produced, click on the 'show' checkboxes at the bottom, then explore what happens as you vary the size of the circles, and their speed of rotation.