John Carlos Baez on Nostr: Some people think medieval astronomers kept adding 'epicycles' to the orbits of ...

Some people think medieval astronomers kept adding 'epicycles' to the orbits of planets, culminating with the Alfonsine Tables created in 1252. The 1968  Encyclopædia Britannica says:

"By this time each planet had been provided with from 40 to 60 epicycles to represent after a fashion its complex movement among the stars."

But this is complete bullshit! Medieval astronomers did *not* keep adding epicycles. They computed planetary orbits using Ptolemy's method, developed way back in 150 AD. This method uses just 6 circles and 6 epicycles - nothing like Britannica's ridiculous claim of between 240 and 360 epicycles.

That's right: Ptolemy got a good fit to planetary orbits using one circle and one epicycle each for the Sun, Mercury, Venus, Mars, Jupiter and Saturn. It's not much worse than what we do now: use one ellipse each for Mercury, Venus, Earth, Mars, Jupiter and Saturn.

In Ptolemy's model, the circles weren't centered on the Earth. They were offset, like in the gif below, where the big green dot is the Earth.

This gif was created by Richard W. Pogge, Distinguished Professor of Astronomy at Ohio State, and you can see more here:

https://www.astronomy.ohio-state.edu/pogge.1/TeachRes/Artwork/Anims/index.html