Ptolemy went on to write a major document that became known as the
Almagest. In it he used a combination of eccentres, equants and epicycles to represent the movement of
all the planets,
including their variation in speed and retrograde motion.
This diagram shows the movement of an outer planet
as described in the Almagest. The planet moves round an
the centre of which moves
round a referent circle. Earth and an equant point are placed an appropriate distance from the
centre of the referent circle. The centre of the epicycle moves such that it would be in uniform
motion seen from the equant point.
The "Mean Sun" moves round the centre of the referent
circle with uniform motion and the line from the centre of the epicycle to the planet is
maintained parallel to the line from the earth to the Sun. Ptolemy's Almagest
was a major step forward in providing the means to predict the positions
of planets and it remained the most complete theory for the next 1000 years.