And how exactly did we find out how they move?
In: Planetary Science
Okay, here’s an actual ELI5 answer – when the planet formed, all of the pieces had their own speeds and directions, and when they all added up, that came out to the spin of the planet and the speed and direction it was going around the sun. That direction wasn’t perfectly aligned with the sun’s location – the sun is actually 3,000,000 miles closer to us in the winter! And so our orbit gets a little too close and a little too far. It is getting rounder, and will someday be a total perfect circle. One side of the planet will face the sun at all times, and the moon will stay over exactly one spot over the earth in the same way. This is called being *tidally locked* and is a fancy way of saying that even if you get bumped a little by the other planets, you still fall back into those shapes. When that happens, it will have finally wound down into a circle.
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