Ptolemaic Model

The Ptolemaic Model, developed by Claudius Ptolemy in the 2nd century, explains the motion of planets and stars using a system of circles. It envisions Earth at the center, with planets moving in small circles called epicycles, which in turn move along larger circles called deferents. This complex arrangement accounts for the observed retrograde motion—when planets appear to reverse direction—by adjusting the sizes and speeds of these circles. Although it accurately predicted planetary positions at the time, it was later replaced by models based on heliocentrism, where the Sun is at the center.