Copernicus
Is it the sky that moves, or us?
That time, that place
He was a canon at Frombork Cathedral on the Baltic. His day job was church administration; he also practiced medicine and wrote on currency policy. The heavens were a lifelong sideline.
Why this question
Ptolemy's system worked. The trouble was the machinery bolted on to make it work: circles riding circles, an offset center, and yet another point from which the motion is measured as uniform.
That last device rankled. It quietly violated the very principle that heavenly motion is uniform and circular. What made Copernicus uncomfortable was not inaccuracy but this fudge.
What was found
Put the Sun at the center and one headache dissolves on its own: the apparent backward motion of the planets. As Earth on an inner orbit overtakes an outer planet, that planet appears to slip backwards. No extra device required.
There was a bonus. The order and distances of the planets fall out uniquely. In Ptolemy's system you could arrange them almost any way and still calculate; here there was only one answer.
Yet he could not give up circular motion. Small circles were still needed, and his accuracy was not noticeably better than Ptolemy's.
The Earth stands still at the center of the universe
That retrograde motion and planetary order fall out of a single picture together
The book appeared just before his death, carrying a preface an editor had slipped in: that this was merely a hypothesis for ease of calculation