Apollonius of Perga
The apparent motion of the heavenly bodies with respect to time is cyclical in nature. Apollonius of Perga realized that this cyclical variation could be represented visually by small circular orbits, or "epicycles", revolving on larger circular orbits, or "deferents". Hipparchus calculated the required orbits. Deferents and epicycles in the ancient models did not represent orbits in the modern sense.