Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Eudoxus' system


Using an onion as a model of Eudoxus' system of the Universe.

Apparatus and materials


Health & Safety and Technical notes

Read our standard health & safety guidance



Eudoxus' model of the universe shaped like an onion

Eudoxus needed four spheres to account for the motion of planets.

An onion can be shown and sliced open, to illustrate Eudoxus' scheme of many concentric spheres.

Teaching notes

1 In Eudoxus' system the Sun and Moon had three spheres each, the planets had four spheres each and the stars shared one sphere. All the spheres were spinning steadily around different axes. The combinations of these motions succeeded in imitating the actual motions of the Sun, Moon and planets across the star pattern.

2 One of the problems which Eudoxus (about 410-350 BC) was trying to solve was that the planets do not move steadily along a circle amongst the stars. They show retrograde motion with a varying speed. Also the speed of the Sun and the Moon varies around their orbits. Eudoxus was the greatest mathematician of his time. He was very clever at the geometry of solid space and he saw how to imitate the planet's looped paths by giving each planet four spheres.
This experiment was safety-checked in April 2007