Apollonius of Perga
 Greek astronomer and geometer c. 262 BC - c. 190 BC, Latin Apollonius Pergaeus. Noted especially for his writings on conic sections. His studies on conics influenced many scholars after him, including Ptolemy, Francesco Maurolico, Johannes Kepler, Isaac Newton, and René Descartes. Apollonius gave the names ellipse, parabola, and hyperbola to the conic sections.
Astronomer and mathematician Eudoxus of Cnidus 408 - 355 BC wrote of the concentric spheres theory, which was based on the Moon, the stars and the planets orbiting the spherical shells.
The weakness of the theory was that the celestial bodies appeared to move further away when the diameters of the Sun and Moon were measured. The dimensions and brightness of the celestial bodies turned out to vary. This thinking led away from fixed spherical shells. Apollonius Bergalalainen is the first known person in history to propose the epicycle theory of planetary orbits. Epicycle meant that the planets revolved around the Earth as a center, but at least some of the orbiting planets had an epicycle, a deviation from a circular orbit. The theory could explain the apparent motion of the planets and the varying speed of the Moon.
Apollonius of Bergala developed geometry, and Hipparchus of Nicaea continued his work on eccentric circles (ellipses) in the following century. He devoted himself especially to observing the movements of the sun and moon and compiling tables of their movements.
One form of the geocentric model was the epicycle theory, in which Venus and the Moon orbit the Sun. They follow the Sun without leaving the Sun's vicinity. Today, the theory is concentric spheres, as originally assumed, but the epicycle is a fact and in practice implies Kepler's first law.
The orbit of a planet is an ellipse with the Sun at one of the two foci.
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