text: what did ptolemy, and later, copernicus do to make their models of the "universe" fit the actual observaions?   when comparing the two models of ptolemy and copernicus the main difference between them is that ptolemy puts the earth in the centre of the solar system, with the sun and planets orbiting it; whilst copernicus had the sun at the centre, with the earth and the other planets orbiting it.  as the heliocentric (sun in centre) is by far the more accurate of the two theories (when considering the solar system), copernicus did not have to make any huge changes to the basic "all planets have circular orbits around the sun" as clearly it almost exactly fits the observed phenomena.  ptolemy had some major problems with retrograde motion when observing the planets - at some times of the year the path of the planets across the sky sometimes reverses and the planet goes "backwards" across the sky, only to reverse it's path again later.  ptolemy invented "epicycles" to account for these retrograde motions.  these were where each planet rotates around an off-centre point close to itself (see the second source below for a diagram).  but these epicycles still did not accurately explain why sometimes retrograde motion lasted longer than other times.  copernicus used a heliocentric basis an wrote 7 basic law which accurately described observed phenomena in a simple way.  these were -  1. there is no one centre in the universe. 2. the earth's centre is not the centre of the universe. 3. the centre of the universe is near the sun. 4. the distance from the earth to the sun is imperceptible compared with the distance to the stars. 5. the rotation of the earth accounts for the apparent daily rotation of the stars. 6. the apparent annual cycle of movements of the sun is caused by the earth revolving around the sun. 7. the apparent retrograde motion of the planets is caused by the motion of the earth, from which one observes.  however, copernicus' theory could not be proved using the equipment of the time (early to mid 1500's) and the long-term use of the ptolemy system, combined with the religious implications of the heliocentric system meant that few people accepted the theory as being correct.  it took until galileo's observations of the phases of venus (around 1610) before there was any clear observational difference between what was predicted by the ptolemy model and the copernican model.  these was swiftly followed by galileo's observation of 4 moons of jupiter, another blow to ptolemy.