Apollonius of Perga (c.262–c.190 BC)
Apollonius of Perga was a highly influential Greek mathematician and astronomer, born in a region of what is now Turkey, who became known as the "Great Geometer." In his famous eight-part work On Conics, he introduced such terms as "ellipse," "parabola," and "hyperbola" – the conic sections that, as we now know, describe the shapes of various types of orbit. Euclid and others had written earlier about the basic properties of conic sections but Apollonius added many new results, particularly to do with normals and tangents to the various conic curves. In particular, he showed that the conic curves can be obtained by taking plane sections at different angles through a cone.
Apollonius also helped found Greek mathematical astronomy. Ptolemy says in his Syntaxis that Apollonius introduced the theory of epicycles to explain the apparent motion of the planets across the sky. Although this isn't strictly true, since the theory of epicycles was mooted earlier, Apollonius did make important contributions, including a study of the points where a planet appears stationary. He also developed the hemicyclium, a sundial with hour lines drawn on the surface of a conic section to give greater accuracy. See also Greek astronomy.
One of the most famous questions he raised is known as the Apollonius problem. He also wrote widely on other subjects including science, medicine, and philosophy. In On the Burning Mirror he showed that parallel rays of light are not brought to a focus by a spherical mirror (as had been previously thought) and he discussed the focal properties of a parabolic mirror. A few decades after his death, the Emperor Hadrian collected his works and ensured their publication throughout his realm.