Aristarchus
At the center — Earth, or the Sun?
That time, that place
Born on Samos, he worked in Alexandria. The common sense of his day was clear: the Earth sits still at the center and the Sun, Moon, and stars turn around it. Nothing underfoot feels like it is moving, so the belief came naturally.
Why this question
He first tried measuring sizes. When the Moon appears exactly half lit, Earth, Moon and Sun stand at a right angle. Measure the angle between Moon and Sun as seen from Earth, and you can compute the ratio of distances. His instruments were crude and the numbers came out far too small, but the direction was unmistakable: the Sun is much larger than the Earth.
What was found
Then something is odd. The far larger body circling the far smaller one? So he turned it around: the Earth goes about the Sun.
The objection came at once. If Earth swings around so wide a circle, the stars should shift a little with the seasons — and they never did. His answer was that the stars lie unimaginably far away. That answer was also correct.
The Earth stands still at the center of the universe
The angle between Sun and Moon measured at half phase
It was scarcely accepted. The stellar shift he predicted was not measured until 1838