# Ptolemy's Explanation of Why Planets Move at Different Speeds

## Transcript

My name is Eric Loberg, the director of here at the Taylor Planetarium at the Museum of the Rockies and I was going to give Ptolemy's explanation of why planets move at different speeds. Ptolemy was an ancient Greek mathematician and he like a lot of the Greeks thought that the earth was in the center of the entire universe, but especially our solar system. The earth was in the middle, then the moon, Mercury, Venus, the sun, Mars, Jupiter and Saturn. It's the same as our system today with the earth in the middle and sun farther out. The ancient Greeks had figured out how long objects took to go around the sun. They just still thought they were going around the earth. They were right about the moon, and so the moon is closest to the earth, Mercury and Venus were the next quickest to go around the sun, so they thought they were going around the earth quicker. And then the sun, Mars and farther out Jupiter and Saturn. So they had our order pretty much right except for the earth and the sun were reversed. The problem with the system was once and a while the planets move and slow down along the star field and this is called retrograde. These planets night after night will slowly move, night after night along the path and then eventually in this path the planets will slow down, go backwards and speed up again. And this is a pattern, this is a path called retrograde. Why this is happening is earth is actually going around the sun and we are catching up with the outer planets and as we catch up and speed around the planets appear to go backwards. But this did not match the model that Ptolemy had. He fixed this by putting in what we call epicycles instead. He put small little rotations in all of these planets and the planets not only moved on their orbit, they made small little circles called epicycles around there. And so adding these epicycles was very important to Ptolemy's model to explain the retrograde motion as well where it goes backwards and slows down in the sky. Instead of just going around these orbits the planets now also moved along the epicycles, circles inside of circles. This model lasted until Copernicus came along and put the sun in the middle of our solar system as we know it is today. But this model lasted almost 1500 years and was very accurate. So Ptolemy was able to explain the motion of planets and how they changed by using epicycles, the small circles. I'm Eric Loberg with the Taylor Planetarium at the Museum of the Rockies.

J. Eric Loberg is the manager, lecturer, and program planner at the Taylor Planetarium at the World, a renowned museum of the Rockies in Bozeman, Montana.