- a system elaborated by Ptolemy and subsequently modified by others, according to which the earth was the fixed center of the universe, with the heavenly bodies moving about it.
- the theory of planetary motion developed by Ptolemy from the hypotheses of earlier philosophers, stating that the earth lay at the centre of the universe with the sun, the moon, and the known planets revolving around it in complicated orbits. Beyond the largest of these orbits lay a sphere of fixed stars
