# 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.