How did copernicus come up with the heliocentric idea?
He didn’t do experiments if that’s what you mean. Mostly it was math. Because the Earth is orbiting the Sun along with the other planets, the planets don’t appear to follow a straight line in the sky. They mostly move forward against the background of the stars, but sometimes turn and move the other way. This retrograde motion is why they are called planets, which comes from the Greek word for wanderer, and why the Greeks thought they were associated with gods, because nothing else in the sky changed direction. In order to build a model of the solar system that produces retrograde motion, there have to be all kinds of tricks built in. Circles in circles and other circles. Wikipedia has a drawing of the “epicycles” that were used to explain retrograde motion. I’ve put a link in the sources. However, although Copernicus replaced the geocentric model with a heliocentric model, he still needed epicycles. This is because he insisted that the orbits of planets would involve circles. There wa
