Slide 4 of 8

Unfortunately, more careful observation of the motion of the planets complicated this simple and elegant picture. For example some of the planets were observed to retrace their steps in the sky at certain times of the year. This so-called ``retrograde motion'' could not be explained in terms of simple circular motion around the Earth. However, the ancient astronomers were very devoted to the principle that the circle had to be the basis for celestial motion and cleverly found a way to retain this view. They realized that if they allowed the planets to move on circles within circles (epicycles) they could in fact fit the retrograde motion of certain planets. Unfortunately, the more accurate the observations became, the more circles within circles needed to be added in order to fit them, until the elegance and simplicity of the underlying geometrical shape was completely overshadowed by the complexity of the model. Moreover, the ``epicycle'' explanation for the motion of the planets lacks predicitive power. Any type of orbit can be fit with this model simply by adding a sufficient number of epicycles. Thus although epicycles could describe celestial motion, they did not in any sense constitute what we would think of today as a scientific theory. Ironically, once Copernicus pointed out that the Sun and not the Earth was the center of planetary motion, the orbits of the planets were revealed to be very nearly circular after all.

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