Planetary Systems

As we all know, the Polish astronomer Nicolas Copernicus' book De revolutionibus orbium coelestium - On the revolutions of the heavenly spheres - was published in 1543 and announced to the world that a better way to explain the apparent motions of the Sun and planets was to take the sun to be at the centre of the universe instead of the Earth. It is perfectly possible to take the view that the Earth is at the centre. If you do so, however, you have the problem of explaining why it appears that the planets seem to move in one direction for a bit, then stop, then reverse direction, then stop, then reverse again to move again the original direction, the cycle then repeating. This is made all the harder if you accept the Aristotelian view (but one that surely pre-dated him) that the orbits of heavenly bodies are circular.

The Greek astronomer Ptolemy (c.90 - c.168 A.D.) came up with an ingenious way of making sense of this. The planets move in circles but the centre of the circle itself moves. Consider the motion of a pod on the London Eye. Now replace the pod with a mini-London Eye and consider the motion of one of its pods. You could indeed then have a mini-mini-London Eye...and so on, to get ever more complex patterns of motion.

Copernicus' system was much simpler: no 'wheels within wheels'. But it didn't persuade everyone. The Danish astronomer Tycho Brahe (1546-1601 A.D.) proposed a system in which the Earth is central. The Sun and the Moon revolve around the Earth. The other planets revolve around the Sun. Brahe couldn't accept that the Earth moved because, if it did, then the pattern of the stars should look different at each of a six month period (when the Earth would have moved through half a complete orbit). The phenomenon is called parallax. You can demonstrate it with by holding out a finger before you and viewing it with just your left eye, then your right. The position relative to the background will change because your eyes are in different positions. In fact the positions of the stars do change, but it is too slight to be seen without a good telescope.

This nice on-line simulator allows you to compare the Tychonian and the Copernican systems. There's a not-quite-as-nice illustration of the Ptolemaic system here and one whose parameters you can modify here.