Nicholas Copernicus
De Revolutionibus Orbium Coelestium
On the Revolutions of the Heavenly Spheres

• Polish, studied in Cracow
• studied in Italy for ten years (law, mathematics, medicine)
• returned to Cracow as a cleric
• worked on manuscript about motions of the planets
• decided to publish at urging astronomer friend Rheticus
• saw the proofs on his deathbead

He put the sun at the center of the Copernican system , in contrast to the the Ptolemaic system

• Predictions obtained with the Ptolemaic system did not quite match observations.
• Simplicity
  
  • [others have considered that the earth moved, even though that seems absurd] "... I considered that I too might well be allowed to try whether sounder demonstrations of the revolutions of the heavenly orbs might be discovered by supposing some motion of the earth ... I found after much and long observation, that if the motions of the other planets were added to the motions of the earth, ... not only did the apparent behavior of the others follow from this, but the system so connects the orders and sizes of the planets and their orbits, and of the whole heaven, that no single feature can be altered without confusion among the other parts and in all the Universe. For this reason, therefore,... have I followed this system."
  • "In this orderly arrangement there appears a wonderful symmetry in the universe and a precise relation between the motions and sizes of the orbs which is impossible to obtain in any other way."
• In contrast, the Ptolemaic system is complicated, especially with its equants.
  • "...the planetary theories of Ptolemy and most other astronomers, although consistent with the numerical data, seemed... to present no small difficulty. For these theories were not adequate unless certain equants were also conceived; it then appeared that a planet moved with uniform velocity neither on its deferent nor about the center of its epicycle. Hence a system of this sort seemed neither sufficiently absolute nor sufficiently pleasing to the mind."
• Still... to get the motions just right, Copernicus needed some small epicycles and spheres not centered exactly on the sun.

Why? Remember Luther (1517). The church had serious problems and could see new ideas as dangerous.