During the late middle ages, the prevailing theory of the solar system was based on a geocentric model, in which the earth was the center of the universe. According to this model the earth was surrounded by spheres, in which were the planets, the sun, and the stars. The planets were perfectly spherical and orbited in perfect circles at a constant speed. However, this model ran into difficulties. For example, at certain times of the year the orbit of mars and the other known planets seemed to reverse in direction, so epicycles were added to account for these faults in the model. Eventually over 80 epicycles were added to the geocentric model.
The 15th-16th century astronomer, Nicolaus Copernicus, published in his work, On the Revolutions of the Heavenly Spheres, a theory in which the sun was at the center of the universe. Copernicus was hesitant to publish this work for fear of ridicule by fellow astronomers, but finally decided to do so in 1543. The heliocentric model was a better way of accounting for the motion of the planets, and Copernicus was able to reduce the number of epicycles. Johannes Keplar expanded in the 17th century by saying that the orbits of the planets were not perfectly circular, but elipses, and that the planets increased in speed when they approached the sun.
Galileo Galilei was an Italian astronomer who had great influence over modern science. He overturned the objection to the heliocentric model that stated that if the earth orbited the sun, then the moon would be left behind, by observing that moons also orbited Jupiter. He also observed that the moon had large craters scattered about it, overturning the theory that the planets were perfectly spherical.
Galileo urged the members of the Church to reinterpret Scripture to come into accordance with his theories, which made him unpopular with them. The Church was also under pressure to deny the findings of Galileo because the Protestants, the primary enemies of heliocentrism, were using the Church’s indecision on the issue to claim that the Catholics were not in accordance with Scripture. It was not necessarily the idea of helicentrism that the Church rejected, as Pope Gregory XIII used Copernicus’s ideas to reform the calendar. Virtually all condemnation disappeared after Galileo’s death, with his books being removed from the banned list in the 18th century.
These men contributed greatly to our current understanding of the universe. They, through observation and calculation, were successful in overturning the Aristotelian and Ptolemaic theory of a geocentric solar system.