Hypothesis: Ptolemy


We’ll start with the model of Ptolemy, who puts the earth in the center of the universe. Remember the nightly observation in the last video? As the stars move overhead during the night, the image of a sphere is inescapable. Based on the idea that the stars were on a large sphere, Ptolemy stuck the earth right in the middle of it, held in place by all that air out there. We don’t feel the earth move and there’s nothing stiller to find in our experience than the ground itself. So, Ptolemy thought that all of the stars move around the earth every single day, and the proper motion of the sun, moon, and planets was added to that common, first motion of the entire heavens.

You may be wondering: “Couldn’t the earth be at the center of the universe and still spin? Wouldn’t that make things easier?” Well, no, that wasn’t possible according to Ptolemy, because then if you jumped up in the air, or fired an arrow, the earth would spin out from underneath it, and you and your arrow would land in another continent. Here’s what Ptolemy wrote about the earth moving in space:

“And if it [the earth] had some one common movement, the same as that of the other weights, it would clearly leave them all behind because of its much greater magnitude. And the animals and other weights would be left hanging in the air, and the earth would very quickly fall out of the heavens. Merely to conceive such things makes them appear ridiculous.”

And here’s what he had to say about spinning:

“For us to grant these things, they would have to admit that the earth's turning is the swiftest of absolutely all the movements about it because of its making so great a revolution in a short time, so that all those things that were not at rest on the earth would seem to have a movement contrary to it, and never would a cloud be seen to move toward the east nor anything else that flew or was thrown into the air. For the earth would always outstrip them in its eastward motion, so that all bodies would seem to be left behind and to move towards the west.”

Now, back to the planets. Remember the loop that we saw the planet make in its motion over time? How could Ptolemy account for that? Well, instead of the planets simply moving around the earth, Ptolemy added a second circle as well. The first circle is called the deferent, and on it spins an epicycle, on which the planet itself is seen. With this epicycle, Ptolemy accounted for the loops made by the planets.

As we grow the epicycle to the appropriate size, it makes the apparent loops the planet traces out, and even makes the planet come closer to the earth during the loopings, and, indeed, Mars appears at its brightest during its backward motion.

Now, while the model you see here has specific sizes for the orbits and epicycles, in order for each planet to have its own sphere, Ptolemy’s mathematical astronomy work didn’t actually include those distances. Since he only cared about where you’d see a planet, and not where it actually was, he only gave ratios between the orbits and epicycles for each planet. So, if I adjust the orbit of Mars by increasing the size of the deferent and epicycle together, it will not change where Mars appears to be, and will still be an equally valid representation of Ptolemy’s model. After all, his only job was to “save appearances.” Now, watch what happens when I adjust all the orbits in a certain way. First, the outer planets: Mars, Jupiter and Saturn. I’ll change the orbits so that the epicycles are all the same size.

(They watch what happens.)

Notice anything in particular? Yes, the epicycles all point the same way. Now, let’s add in the sun. When you look at it like this, with all the epicycles aligned with the sun, it seems pretty zany to give the sun no role whatsoever in the motion of the planets, especially since Ptolemy’s predecessors in Greece had already put forward the sun being at the center, but we’ll stick with what Ptolemy said for now. For him, each planet had its own particular movers. There was nothing physically common to the proper motion of the different planets (excluding the daily motion of the entire heavens around the earth).

And now, the inner planets: Mercury and Venus. For these two, we’ll adjust the deferents until they are the same size as the sun. Sure looks like they’re going around the sun, doesn’t it? This is a valid representation of Ptolemy’s ratios. Did he really not realize this?

Oh, and don’t forget about the daily motion:

Doesn’t this seem silly? Here’s what Benjamin Franklin had to say about this scheme:

“Ptolemy is compared to a whimsical Cook, who, instead of Turning his meat in roasting, should fix That, and contrive to have his whole Fire, Kitchen and all, whirling continually round it.”

Next: Ptolemy's Equant