Why was Ptolemy's Earth-centered system of epicycles taught throughout Europe for over a thousand years?

Answer 1

Because plain circular orbits could not explain the way the planets move. The philosophers said that the circle was the perfect curve, and Ptolemy's model used circles and epicycles.

An epicycle is a small circle whose centre travels round a larger circle, and the planet then travels round the epicycle. Extra epicycles were added to account for things like the eccentric orbits (which are now recognised as elliptical), and the inclination of the orbits.

However Ptolemy's model represented the movements of the planets pretty accurately which is why no-one thought to challenge it for 1400 years. Ptolemy's model was also in use in planetariums until modern times, when computer driven planetariums took over.

In Ptolemy's model both Venus and Mars have very large epicycles, which we now understand are there to account for the Earth's movement round the Sun.

Answer 2

Because they could see that to assume circular orbits gave the wrong positions for the planets. So they added epicycles with the centre of the epicycle travelling round a circle, while the planet itself was situated on the smaller epicycle.

The complete model had several epicycles for each planet, each one superimposed on the previous one, but the basic epicycle was the one was the one that reproduced the effect of the distance variation (we now know it is due to the elliptical nature of the orbit). While the centre of the epicycle moved round the main orbit (the deferent), the planet did not rotate around the epicycle. This produced the overall effect of a circular orbit with the Sun displaced from the centre of the circle, which is in fact a good model for the shape of most planets' orbits, good enough to delay the discovery of elliptical orbits by well over a thousand years.

In each orbit, further epicycles were added to the model to allow for the effect of the planets' orbits not all lying exactly in the same plane; and other effects.

Answer 3

Ptolemy's Earth-centered system of epicycles was widely accepted and taught for so long because it provided accurate predictions of planetary positions and movements, based on the observations available at the time. Additionally, the Church and other authorities supported this system because it aligned with the religious belief of Earth being the center of the universe. It wasn't until Copernicus and later astronomers challenged this model with heliocentrism that a shift occurred.

Answer 4

It was both scientifically accurate and also approved by the Church.

Answer 5

It was his way of explaining why it appeared that planets were moving backwards when Earth passed by them