Analyse and compare the information you have processed to assess the relative accuracy of the two models at predicting future astronomical events, such as the position of the planets.

Essay by vivivivi April 2008

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Ptolemy’s Model became highly successful in predicting changing positions of the stars and planets during the ancient times. But the predictions made with Ptolemy's model of the planetary position and precessions of the equinoxes were not quite consistent with the best available observations. Because of a particular inconsistency, astronomers made minor adjustments in the Ptolemaic model to account for the inconsistency but eventually the web of complexity resulting from the minor adjustments was increasing more rapidly than the accuracy, so a discrepancy corrected in one place was likely to show up in another place.

Copernicus’ model is able to account qualitatively for the observed motions of the stars, the Moon and the position of the planets. Copernicus wanted to increase the accuracy of his model by inserting small epicycles for the celestial objects to move on. This was necessary because Copernicus assumed (like Ptolemy) that the planets move at a constant speed.

But the observed motions did not correspond exactly to planets moving at constant speed. The epicycles of Copernicus were smaller than those of Ptolemy and the motion of the planets around them was much slower so that each planet moved in an elongated circle way.

But even with his epicycles, Copernicus’ model resulted in errors in predicting planetary positions of about 2 degrees over a few centuries of planetary motion. This is the same error in Ptolemy’s model. There was no definitive observation that could be made that would decide which of the two competing models fit the data more accurately. So to compare the two models, it is based on the criteria of the data fitting. For example, observing stellar parallax would have clearly shown that the Earth moves and thus supported the heliocentric model.

Both models were however able to explain why the observed motions of...