What are Ptolemy's theory of the orbit?

Answer 1

Ptolemy used circles and epicycles for his model.

The basic orbit is a large circle, the deferent. Then there is a smaller circle called an epicycle with its centre going round the deferent, and the planet travels round that epicycle. That is the simplified model that accounts for planets that are, as we now understand, going round the Sun in circular orbits (in a simplified view).

The epicycle is there to allow for the fact that, as we now know, the Sun is at the centre. Ptolemy's aim was to model the planets' positions as seen, and his model does that quite accurately.

Take Venus's orbit, with Venus going round in 225 days compared to the Earth's 365 days. In Ptolemy's model the Earth is fixed, so Venus's deferent is a circle with the centre of the epicycle going round every 365 days. The deferent has a radius of 1.000, let's say, and then the epicycle has a radius of 0.723, a little smaller, with Venus travelling round the epicycle in 225 days. It's a good model of what actually happens, and this is the model that was used in planetariums until the days of computers. We now now that if the Earth's orbit has a mean radius of 1, then Venus's orbit has a radius of 0.723.

Fred Hoyle the astronomer pointed out that the deferent and the epicycle are interchangeable without making any difference to the model (because, he said, addition of complex numbers is commutative) . But by convention the largest one always seems to be called the deferent.

Ptolemy's model represents the movement of Venus fairly accurately and the question of whether the Earth or the Sun was at the centre was not properly answered until discoveries of the law of gravity and the laws of motion, which was well after the times of Copernicus, Tycho, Galileo and Kepler. Then it was realised that the Sun must be at the centre because it is so much more massive.

Ptolemy's theory was in fact more complex than explained in this article, and it also allows for what we now call the eccentricity and the inclination of each orbit.

Answer 2

Ptolemy's theory of the orbit, known as the Ptolemaic system, posited that the Earth was at the center of the universe and all celestial bodies, including the Sun, Moon, and planets, orbited around it in perfect circles. To account for the irregular motion of planets, Ptolemy introduced the concept of epicycles, which were smaller circles that planets moved on while still orbiting Earth. This geocentric model was widely accepted for centuries until Copernicus proposed the heliocentric model.