In the Hipparchian and
Ptolemaic systems of
astronomy, the
epicycle (from , literally
on the circle, meaning
circle moving on another circle) was a geometric model used to explain the variations in speed and direction of the apparent motion of the
Moon,
Sun, and
planets. In particular it explained the
apparent retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from Earth.