Eudoxus of cnidus biography of william hill

Importance to Astronomy

Eudoxus was the first person to devise a model that could explain the retrograde motion of the planets in the sky.

(Keep in mind that there was not a great deal of real accurate data regarding the positions of the planets at the time.) Eudoxus was a gifted mathematician and was able to show that using three concentric spheres, one could arrange the spheres in such a way that a planet attached to one of the spheres could be made to travel around the common center, making periodic retrograde motions.

Because the spheres are all concentric, this is often called the "homocentric" model of planetary motion.

So how well did the model work?

Eudoxus of cnidus biography of william hill

Not very well, unfortunately. It had two glaring errors: the shapes of the retrograde loops in this model were always the same and it does not account the increase in brightness of the planets during retrograde motion. The success of the model, and its subsequent adoption by Aristo