You know, because you've been told, that the Earth revolves around the Sun. You also probably know that planets other than our own have moons, and the way to test to see whether or not something is true is by experimenting. Thousands of years ago, these things were not widely known. The heavens above were anyone's guess, and the way things were was just the way the gods had made them. It was felt there was no need to truly understand them or put them in any kind of order.