Nicolaus Copernicus (1473-1543) was an astronomer who said the planets circle the Sun — not the Earth as Ptolemy had said. He laid out his theory in a book called “On the Revolutions of the Celestial Spheres”. He came up with the theory in the early 1500s, but the book did not come out till 1543 when he was on his deathbed.
Copernicus was not the first to say this: back in ancient Greece Aristarchus had said the same. But it could not be proved true till the telescope was invented in the early 1600s.
The theories of Copernicus and Ptolemy both fit the facts known in 1500 equally well. Copernicus’s theory was a bit simpler on paper, but it had one great drawback: the known physics of the time did not support it. Ptolemy’s theory made physical sense, Copernicus’ did not.
The known physics of the time was that of Aristotle. He said the universe was made up of five elements.From heaviest to lightest they were: earth, water, air, fire and quintessence. Things move here and there but in the long run the elements tend to their natural places. The heavier an element the more it tended to move towards the middle of the universe. That is why Earth is made up of earth, with water on it and air above it. It is why fire goes upwards. It is why there is no quintessence on Earth – it is all up in the heavens.
Heavenly bodies — the Sun, stars and planets — were made of quintessence. That is why they are way above the earth. It is also why they always go in circles round the Earth: since quintessence is so perfect, its motion is perfect. Aristotle said that the perfect motion was in circles.
So in the late 1500s Copernicus’s theory was merely “interesting”, at best a useful fiction that made it easier to work out the positions of the planets. The Church did not even outlaw his book then – that did not happen till 1616.
The turning point came in the early 1600s with Galileo.
First Galileo put Aristotle’s physics to the test and found out it was not as true as everyone had thought. There were things in it that were flat out wrong.
Second, he made a telescope and looked at the night sky. He found that the heavenly bodies were not so perfect after all – another strike against Aristotle. He saw that Venus waxes and wanes just like the Moon and changed size as it did so. Copernicus could explain that, Ptolemy could not. He also saw four moons circling Jupiter – how does that fit into Aristotle’s and Ptolemy’s system?
With Aristotle’s physics in deep trouble and the facts now on Copernicus’ side, his theory began to win the day.
Final victory came with the work of Galileo, Kepler and Newton. Galileo and Newton worked out the new physics and Kepler perfected the theory by having planets move in less-than-perfect circles – in ellipses. By 1700 Ptolemy was history.