What made us think that Earth moves around the Sun?

Question

Trying to observe the night sky for a few weeks, the motion of the Sun and the stars pretty much fits into the Geocentric Theory i.e. All of them move around the Earth.**What then, which particular observation, made us think that it could be the other way around, that all the planets move around the Sun?**

Answer 1

In the case of Mercury and Venus, we can watch them move around the sun. Those two planets are only visible in the sky over sunrises and sunsets, and their way of life is to swing back and forth from one side of the sun (visible over the sunrise) to the other side of the sun (visible over the sunset). Mercury is never more than about 18° from the sun in the sky, and it flip-flops between sunrise and sunset every few months. Venus is never more than about 45° from the sun in the sky, and it flip-flops every couple of years. So it isn't too hard to explain their behavior by saying that Mercury circles the sun in a close, fast orbit, and Venus circles the sun in a larger, slower orbit.

One of the first people in history to guess that the Sun might be at the center of everything in the universe, rather than the Earth, was Aristarchus of Ancient Greece. By paying attention to the phases of the moon, he realized that the Sun was many, many times farther away from us than the moon. (The next time you see a quarter moon in the sky, imagine the triangle in space formed by the Sun, Moon, and Earth. The bright side of the moon faces the sun, implying a nearly right angle at the moon's corner of the triangle. You can see with your own eyes that the Sun and Moon are nearly a right angle apart in the sky, giving a nearly right angle at your corner of the triangle, i.e. the Earth's corner. This gives a very long, skinny triangle, and implies that the distance from you to the sun is much, much, larger than the distance from you to the moon.) However, the moon and the sun appear to be nearly the same size in the sky ... and Aristarchus also realized that if the sun is many, many times farther away than the moon, it must be many, many times larger, as well. This was the first time anyone realized that there might be things in the universe larger than the Earth itself. And shouldn't the biggest thing be in the middle?

This remained merely a guess, however, for 2000 years. The story of how Copernicus, Galileo, and Newton demonstrated that the Sun is the center of the "Solar System" is one of the most famous in science, but in a nutshell: Copernicus showed that a heliocentric arrangement could explain retrograde motion, among other observable things, in a very neat way, and then Galileo and Newton showed how gravity and the laws of motion could explain why the heliocentric model works.