When you go outside at night, on a clear night away from all lights, you see the sky the same way the ancients did: full of stars.
Now, if you looked up periodically, you would find that the sky appears to rotate! Some constellations rise while others set, and one point — either due north or due south depending on your hemisphere — appears to not move at all. With the advent of time-lapse photography (and go here for a fantastic video), we can see that the sky does something like this:
So there’s some pretty good evidence, right away, that either the Earth is rotating or the entire sky is rotating. But there are a few bright objects in the sky that don’t make this same motion every night. A few of them move to a different point in the sky each night. The Ancient Greeks called them planetas, or wanderers.
If you look at a planet, like Mars, relative to the other stars, it appears to zip through the sky in one predominant direction. But every once in a while, it does something bizarre. It stops, goes backwards, stops again, and then resumes its original direction. This — today — is wikipedia’s featured picture of the day:
So, how do you explain that one, folks? Well, Ptolemy came up with a simple, elegant, and completely wrong explanation. He said that instead of Mars moving in a circle around Earth, it moved on a “circle within a circle”, allowing it to sometimes go backwards:
It wasn’t until nearly 1500 years later, when Copernicus realized that if an inner planet moved faster than an outer planet, it would appear that Mars moved backwards from the point of view of Earth.
So that’s the cause of the apparent “retrograde motion” of Mars. What the history books don’t tell you? By time Copernicus came along, Mars’ orbit had been so carefully studied that geocentric modelers of the Solar System had placed seventy-eight epicycles on Mars’ orbit!
And, much like you, I wonder what blind alley we’re inadvertently treading down, adding epicycles to, all because we don’t have the proper perspective? (My guess is dark energy, but who knows?)