Welcome back!

In the last two episodes, we looked at some fundamental ideas that will become important for understanding how the Greeks approached astronomy. We saw that they proved the Earth is a sphere, and we learned how they thought about motion—especially how celestial bodies move. Now it’s time to start putting these pieces together.

Though the Greeks were not the only civilization to understand that the Earth is a sphere, their desire to come up with a mechanical model of the cosmos was unique. They wanted a physical explanation for why the sun, moon, planets, and stars move the way they do, not just a description of their motions.

This search for a physical explanation led to one of the most influential ideas in the history of astronomy: the celestial spheres. Plato hints at this idea in the Timaeus, imagining the heavens as a system of rotating circles.

The basic idea behind the spheres was as follows:

• The Earth is a sphere.
• The heavens appear to move in circles.
• So perhaps each celestial body is carried around the Earth by a larger, invisible sphere that spins at a steady rate.

This was the Greeks’ first attempt to explain the motions of the heavens with a physical system—the first true mechanical model of the cosmos in Western history.

Now, I don’t think it’s hard for us to imagine this for the stars. We’ve already seen that the stars appear to move together as a single unit. We’ve also seen that one star—Polaris—doesn’t appear to move at all. And we’ve already imagined the stars as if they were studded into a giant black sphere surrounding the Earth and rotating around the pole defined by Polaris.

So one way to visualize it is like this:



Here you can see two poles representing the north and south poles. The Earth sits at the center, surrounded by a vast sphere of stars moving from east to west. From your position on Earth, it would appear as if the stars were rising in the east and setting in the west.

This idea fits neatly with the Greek view of celestial motion: a perfect sphere moving at a constant rate in a perfect circle, forever. And that is exactly what we observe. Night after night, the stars appear to circle the Earth at a constant rate, never changing.

So this sphere is relatively easy to wrap our heads around—but things are about to get much more complicated. The Greeks were committed to the idea that all celestial spheres must move in uniform circles around the Earth. For the stars, this works beautifully. But what about the sun, moon, and planets?

Take the sun. It would be simple if the sun behaved exactly like the stars, just moving slightly slower. But we’ve already seen that it doesn’t. The sun moves east to west, yes—but its path across the sky also changes over the course of the year. In the summer, it climbs higher; in the winter, it stays lower. So how are the Greeks going to explain that with perfect, uniform circles?

Well, it’s time to introduce our first major astronomer: Eudoxus. He created the first geometric model of the solar system. His goal was to use spheres, all moving at constant and uniform rates, to explain the motions of the celestial bodies. And in our next episode, we’ll see how he attempted to do this for the sun.

See you then!