The Ptolemaic Universe

Ptolemy Retrograde Motion.jpg

Claudius Ptolemy lived about five centuries after the Greek philosopher Aristotle’s time. Aristotle’s model for the universe—the first geocentric model, with Earth at the center—was still widely accepted, and Ptolemy sought to improve it.

Ptolemy was one of the first of the ancient Greeks to be a true astronomer and mathematician, rather than a philosopher.

Where Aristotle, Plato, Thales, and Pythagoras before him had tried to use “pure thought” to understand the nature of the heavens, Ptolemy set about to perfect the geocentric model mathematically.

This was a huge step forward for science as a whole, as science today relies heavily on mathematics.

In Ptolemy’s time, science didn’t really exist yet. The Greeks preferred to just think through problems logically and reasonably, and if the logic they used was based on untrue assumptions…well, no one was the wiser.

But Ptolemy came up with the wonderful idea to line up observations of the sky with mathematics. And even though Aristotle’s view of the universe shackled him, he moved science forward with great strides.

You might wonder how on Earth Ptolemy made a geocentric universe line up with observations. After all, the geocentric universe isn’t true—even the heliocentric (sun-centered) model is wrong, since the sun isn’t the center of the universe.

In fact, while we’re at it, you might wonder how in the world astronomy managed to hold itself back for 2000 years while all the Greek astronomers insisted the Earth was the center of the universe.

It all has to do with something called parallax.

Parallax is a pretty simple thing, and you can see it for yourself with a simple experiment. Find an object relatively close by to focus on, like a nearby building or tree. Just make sure it’s not too far away. You’ll see why in a minute.

Now hold your finger out in front of it, and close one eye.

Now close the other eye. Don’t move your hand.


You’ll notice that even though your hand didn’t move, it appears to move. That’s because your eyes each have a slightly different perspective, since they’re set a little distance apart on your face.

See, the Greeks reasoned that if the Earth moved, then parallax would surely be detected among the stars. When the Earth was in one position, it would be like your left eye being open, and when it moved to the other, it would be like opening your right eye.

But they couldn’t see parallax at all. Why? Because the stars are so far away, there is parallax, but you can’t see it with the naked eye.

Unfortunately for ancient Greek astronomy, the telescope hadn’t been invented yet, so they couldn’t measure that minuscule parallax.

It never occurred to the Greeks that the stars could be that far away; that kind of distance was absolutely unimaginable. Aristotle himself believed the Earth to be less than half as large as it really was.

Now that the Greek preference for the geocentric universe makes more sense, let’s take a closer look at how Ptolemy tried to fix up the geocentric model.

Aristotle’s universe was simple, by Ptolemy’s standards. He described the universe as a set of 55 nested spheres which caused the motion of the objects in the sky. Here’s a simplified diagram:


The problem was, there was one key feature of planetary motion that this model just couldn’t explain.

Planets have a habit of moving a bit backwards.

Yes, you read that right. Backwards.

If you observe, for example, Mars, every night for a few months in a row, you’ll notice what I’m talking about. It literally moves in a flattened loop across the night sky.

Mars Retrograde.jpg

I know, I know, that makes basically no sense at all. You’re probably about ready to dismiss this impossibility right about now; after all, why would Mars ever move backward?

Well, it doesn’t really move backward. There’s a very simple explanation for retrograde motion, and I’ll cover it in a post coming up. For now, you can understand how frustrated the ancient astronomers were.

How could they possibly explain these little star-like objects’ tendency to wander around?

Fun fact, though—the word “planet” literally means “wanderer.” Surprise?

Ptolemy’s suggestion was quite ingenious, if I do say so myself. After all, as long as he was trapped in the paradigm of the geocentric universe, any explanation he came up with had to be a little more complicated than necessary.

He suggested the epicycle.


The epicycle is basically a smaller circle that turns on a larger circle.

A planet, such as Mars, orbits on the epicycle. The epicycle, in turn, orbits around the Earth on a deferent.

The epicycle wasn’t a physical thing. It was meant merely to describe the planet’s retrograde motion.

And it did a good job of it—epicycles made the planets orbit in exaggerated loops around the Earth, almost like flower petals.

You can see how the epicycle did a good job of explaining motion that made no sense in the sky. How was Ptolemy supposed to know that the Earth was actually orbiting with these dots of light around the sun? No one had even suggested the Earth was a planet yet.

But of course, Ptolemy’s model wasn’t accurate. You can’t get accuracy from a model that isn’t even right.

It did a good job at first, but it was like a watch that was slow by a single second—over a long time, the seconds accumulate and your watch is way off. In the same way, the Ptolemaic model accumulated errors until it was just as unreliable as Aristotle’s nested spheres.

People over time tried to fix the Ptolemaic model. After all, it followed the great Aristotle’s teachings, so it had to be right, didn’t it? Ptolemy ended up adding a lot more epicycles that orbited epicycles, just to try to get that retrograde motion right.

The moral of the story here is that the more complex a scientific model is, the more wrong it probably is. (Important note: I’m not talking about models with lots of variables!)

If a model tries to explain something using lots of unnecessary epicycles but can never get spot-on accurate no matter what you try, there’s a pretty good chance it’s wrong.

Generations upon generations of Greek astronomers tried to improve the Ptolemaic model, but no matter what they did, it always accumulated more errors over time. And eventually, something had to give.

So along came Copernicus…with his revolutionary idea that—wait a second, what? The Earth moves around the sun?

Surely not.

That’s what the Greeks said. And they were about to face their greatest test yet—could they break out of their geocentric paradigm and choose observations over Aristotle’s teachings?

Questions? Or just want to talk?

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