Copernicus's Model of the Solar System

The Polish astronomer Nicolaus Copernicus (1473-1543 CE) studied the Almagest assiduously, but eventually became dissatisfied with Ptolomy's approach. The main reason for this dissatisfaction was not the geocentric nature of Ptolomy's model, but rather the fact that it mandates that heavenly bodies execute non-uniform circular motion. Copernicus, like Aristotle, was convinced that the supposed perfection of the heavens requires such bodies to execute uniform circular motion only. Copernicus was thus spurred to construct his own model of the solar system, which was described in the book De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), published in the year of his death.

The most well-known aspect of Copernicus's model is the fact that it is heliocentric. As has already been mentioned, when describing the motion of the sun, moon, and planets relative to the earth, it makes little practical difference whether one adopts a geocentric or a heliocentric model of the solar system. Having said this, the heliocentric approach does have one large advantage. If we accept that the sun, and not the earth, is stationary, then it immediately follows that the epicycles of the superior planets, and the deferents of the inferior planets, represent the earth's orbit around the sun. Hence, all of these circles must be the same size. This realization allows us to break the scale invariance which is one of the main failings of Ptolemy's model. Thus, the ratio of the deferent radius to that of the epicycle for a superior planet, which is easily inferred from observations, actually corresponds to the ratio of planet's orbital radius to that of the earth. Likewise, the ratio of the epicycle radius to that of the deferent for an inferior planet, which is again easily determined observationally, also corresponds to the ratio of the planet's orbital radius to that of the earth. Using this type of reasoning, Copernicus was able to construct the first accurate scale model of the solar system, and to firmly establish the order in which the planets orbit the sun. In some sense, this was his main achievement.

Copernicus's insistence that heavenly bodies should only move in uniform circles lead him to reject Ptolemy's essentially correct equant scheme, and to instead replace it with spurious additional epicycles. Consequently, Copernicus's model of the solar system contains far more epicycles than Ptolemy's. Indeed, the model of Copernicus is more complicated, and less accurate, than that of the Almagest. In this respect, Copernicus cannot be said to have demonstrated the correctness of his heliocentric approach on the basis of observational data.