Epicycles

The Greek astronomer Ptolemy used to think that the Earth was at the centre of the Universe. All the heavenly bodies, including the Sun and the planet revolved around the Earth in circles, because they thought, circles were as close to perfect as geometry could be.
Observations of the planets showed they did not move in circles around the Earth. Planets appeared to move sometimes forward, sometimes backwards, relative to the fixed stars, changing direction. The Greeks were eager to preserve the Ptolemaic model of the Universe, and they invented the concept of the epicycle - a circle on a circle - to account for the apparent change in the direction of the planets.

The deviation of planets  from circles around the Earth can be explained to a good approximation by adding a small circle to the large circular orbit.

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