The Epicycle theory of Ptolemy (AD 150)

Ptolemy proposed this model to try to explain the motion of a planet.

The planets were thought to travel round the Earth on the small circle called an EPICYCLE whose centre moved round the large circle. The combined motions of P and D accounted for the backward or retrograde motion of the planet P which an observer on Earth would see.

As the motions of the planets were measured more and more carefully the Epicycle theory became more and more complicated.

The looping motion of Jupiter as seen from the Earth

If Jupiter (or indeed any of the planets further from the Sun than the Earth) is viewed over a few weeks it shows a looping motion against the background of the stars. Indeed there are times when it appears to be moving backwards in its orbit. This apparent backward motion is called retrograde motion. It was a real problem for the early astronomers who believed that the Earth was the centre of the Universe. Figure 2 shows how the retrograde motion is explained by the Copernican theory with the Sun at the centre of the Solar System. This is the theory that has been proved correct and is accepted today.