The Copernican Revolution

In 1543 a Polish mathematician and astronomer by the name of Nicolaus Copernicus (1473-1543) published a treatise that proposed a radically new model for the structure of the universe. The earth was removed from the center of the universe and replaced by the sun. The planets, earth included, were pictured as traveling around the sun along perfectly circular orbits. The planetary orbits were all nearly coplanar with each other, and all the planets traveled around the sun in the same direction. The copernican model of the solar system said to be heliocentric, as it places the sun rather than the earth at the center of the universe. The moon, however, was still pictured as revolving about the earth, just as it did in the Ptolemaic model. The earth was assumed to rotate about an axis that pointed toward the north star, so that the entire celestial sphere appeared to complete one full turn every day. It was not the sky that was turning, it was the earth.

Mercury was placed closest to the sun, followed at increasing distances by Venus, Earth, Mars, Jupiter, and Saturn. The copernican model readily explains the variable brightness of the planets, since they maintain a fixed distance from the sun, not the earth. It also provides a convenient explanation of the fact that mercury and venus always appear near the sun in the sky. These two planets occupy orbits with smaller radii than Earth`s. The copernican model also provides as explanation of the retrograde motion of Mars, provided one assumes that mars travels more slowly along its orbit than the earth does. At the time that the earth catches up to mars in its orbit, the red planet appears to us to be momentarily traveling backward with respect to the stellar background, for the same reason that a slower moving car appears to the occupants of a faster car to be traveling backward with respect to a distant background at the instant of passing.

There were problems with the copernican model, and by no means did it meet with instant acceptance. There were significant departures from perfectly uniform circular motion (especially in the case of Mars), and later the theorists were forced to add epicycles to the motion in order to get a better fit. The most severe problem was caused by the moving earth. Not only was this offensive to the prevailing religions at the time, but it also did violence to the conventionally understood notions of mechanics that dated from the time of Aristotle (384-322 B.C.). It stood to reason that if the earth were really moving, an object dropped a height would fall far to the west of its release point rather than straight down. For these reasons, the copernican model of the universe was actively suppressed by both the catholic and protestant branches of the christian religion.