When I was a kid, the solar system was simple. There were nine planets and they all orbited in more-or-less circles around the sun. This same sun-and-a-handful-of-planets scheme repeated itself again and again throughout our galaxy, and these galaxies make up the universe. It’s a great story that’s easy to wrap your mind around, and of course it’s a great first approximation, except maybe that “nine planets” thing, which was just a fluke that we’ll examine shortly.
What’s happened since, however, is that telescopes have gotten significantly better, and many more bodies of all sorts have been discovered in the solar system which is awesome. But as a casual astronomy observer, I’ve given up hope of holding on to a simple mental model. The solar system is just too weird.
The Ancients and the Asteroids
It’s probably all Plato’s fault. While all of the ancient astronomers, from the Babylonians to the Egyptians, had noticed that some of the stars seemed to wander around relative to the others, it was Plato who posited that the fixed stars were located on one sphere, and the planets on another. That’s about as simple as you can get.
Ptolemy noticed that some of the planets seemed to wiggle around in their wanderings, and broke the planetary sphere by claiming that planets followed epicycles, and that each planet had its own. Others proposed epicycles on the epicycles, fitting the data better, but making for a very complicated system. Copernicus later managed to shrink the epicycles, explaining the motion of the planets by putting the sun at the center of the solar system. But it wouldn’t be unt ..
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