Apollonius of Perga and Hipparchus of Nicaea developed a new planetary model. The planet travels with uniform motion on a circumference called an epicyle, whose center revolves at uniform velocity along a larger circumference—called the deferent—around the Earth, immobile at the center of the Universe.

The motion of the center of the epicycle represents the mean motion of the planet along the zodiac, while the epicycle accounts for its retrograde motion. When the planet travels along the upper half of the epicyle, it moves in the same direction as the motion along the deferent, and the two motions are therefore added together. When the planet moves along the lower half of the epicyle, its motion, now in the opposite direction, is subtracted from the motion along the deferent. To the terrestrial observer, the planet thus appears to be speeding across the zodiac when it is located in the upper half of the epicyle and to be slowing in the lower half until it stops and, briefly, reverses direction.

Mars, Jupiter, and Saturn move in such a way that the segment joining the planet to the center of the epicycle is always parallel to the line joining the Earth and the Sun. For Venus and Mercury, the center of the epicycle is always located on the line joining the Earth and the Sun; this explains why the two planets never diverge from the Sun by more than a given angle.

Whereas Eudoxus postulated that the planets always remained at the same distance from the Earth, the epicyclical model assumes a continuously varying distance. This arrangement also explains the changing brightness of the planets, which peaks during retrograde motion. According to the model, that is exactly the point where the planets are nearest the Earth.