Claudius Ptolemaeus (better known now as Ptolemy), from around 100-168 AD - was an Alexandrian astronomer, mathematician, geographer, astrologer and music theorist, who published about a dozen scientific theories, of which three are of major importance.
The first was Almagest, which proposed his geocentric model of the Universe.
The second was Geography - a discussion of maps and the Greco-Roman geographic knowledge.
The third was an astrology text.
Ptolemy's Geocentric Model
This had Earth as the centre of the Universe, circled by the Sun and the planets. Ptolemy pulled together the models of ancient Greek and Roman astronomers such as Aristotle, where the Earth, Sun and planets were all spheres that moved in perfect circles. By the 3rd century BCE it was accepted that the Earth was spherical.
In Ptolemy's model the Universe was a set of nested spheres surrounding the Earth and each planet was moved by a system of two spheres, a deferent and an epicycle.
The deferent is a circle whose centre is called the eccentric and is away from the Earth. This accounts for the seasons. The epicycle is a second smaller sphere within the deferent sphere. These movements continue to explain how planets slowed down, stopped and moved in a retrograde direction. This model had been used by Greek astronomers for centuries but Ptolemy added an equant (a point near the centre of a planets' orbit, such that if stood there the centre of the planet's epicycle would always appear to move at the same speed). This predicted various celestial motions to within a maximum error of 10 degrees.
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