Last time, we looked at how the Greeks were able to show that the Earth is a sphere. That’s going to become very important as we move forward. But before we jump into the first real attempts to model the heavens, there’s one big idea we need to understand: how the Greeks thought motion worked.

So far, we’ve been looking closely at how the Sun, Moon, stars, and planets move. But before we can take the next step toward modeling those motions, we need to understand something the Greeks believed about motion in general. Their ideas about what causes things to move—and what things are made of—ended up shaping astronomy for nearly two thousand years. Those ideas will strongly influence the way they try to explain the motions of the heavens.

Up until now, most of what we’ve focused on has been simple observation—tracking how things move across the sky from day to day. The Greeks, though, were not content just to watch. They wanted to know why. And for that, we turn to Aristotle.
Aristotle believed there were two main kinds of motion:

Violent motion

This is motion caused by an outside force.

If I pick up a rock and throw it, the rock is moving because I made it move. The motion is “violent” in the sense that it comes from something external.

Natural motion

This is how a thing moves when it’s acting according to its nature—how it moves when nothing is pushing or pulling it. Aristotle noticed that different kinds of things tended to move in characteristic ways. He wondered whether natural motion might be tied to what each thing is made of.



The Greeks had a much simpler view of matter than we do. They believed everything on Earth was made of some combination of four elements: earth, water, air, and fire.

• Things that looked like earth fell toward the ground.
• Water also moved downward but settled on top of earth rather than sinking through it.
• Air rose.
• Fire rose even more quickly.
• 
  

So Aristotle reasoned: maybe the natural motion of an object depends on which of these elements it’s mostly made of. A rock falls because it’s “earthy.” Smoke rises because it’s “airy.” That sort of thing.

But this raised a big problem when he looked up.

The Sun, the Moon, the planets, and the stars clearly move—but not in the ways any of the four elements do. The Sun and stars look fiery, but they don’t rise away from Earth. The Moon looks like a giant rock, but it doesn’t fall. Instead, all the celestial bodies move in these perfect, steady circles around the Earth, repeating the same paths night after night, year after year.

This motion didn’t fit the system.

So Aristotle had two options:

Either rethink his whole theory of motion, or conclude that the heavens are made of something else entirely.

He chose the second.

A fifth element

Aristotle proposed that the entire heavens—the Sun, Moon, planets, stars, and even the space between them—are made of a special fifth element called aether.

Aether was unlike anything found on Earth.

Its natural motion wasn’t up or down. Instead, its natural motion was uniform circular motion: perfect, unchanging circles that go on forever.

This idea fit beautifully with what the Greeks saw in the sky. The heavens looked pristine and unchanging. Their motions were regular and repetitive. And the Greeks already regarded the circle as the most perfect shape. So the idea stuck.

Over time, three major principles emerged as the foundation of Greek astronomy for nearly two thousand years:

1. Geocentricity — the Earth sits at the center of the universe.
2. Circular motion — celestial bodies move in perfect circles.
3. Uniform motion — they move at a constant, unchanging speed.

Now, you may already see where this leads. Over the course of a single day, this looks pretty good. Everything arcs across the sky in a nice smooth circle. But if you keep watching, things get messy. The Sun shifts its rising point through the seasons and the planets speed up and slow down.

The Greeks knew this. So how did they hold on to their perfect circles and constant speeds while also explaining these irregular motions?

Next time, we’ll look at one of the earliest attempts to solve that puzzle: Eudoxus and his model of the heavens.

Until next time!