Welcome to Episode 13!

As a reminder, I’ll be switching to email-based episodes for the next little while. I’ll still link to YouTube videos when needed, but most of each lesson will be shared by email and available on the Tracing the Sky website.

Today, we’ll begin our journey through ancient Greece. Before we meet our first Greek astronomer, I want to take a moment to talk about how the Greeks thought about the shape of the Earth and about motion in general. In this episode, we’ll focus on what they believed about the Earth’s shape.

So far, we’ve been focused on how the celestial bodies appear to move. We’ve seen that it looks like everything in the sky is circling the Earth from east to west. But what kind of thing are all these objects circling? What shape is the Earth itself?

You may have heard that many ancient civilizations believed the Earth was flat—and that people didn’t realize it was round until Christopher Columbus. You might even picture something like this:



That’s actually a myth! In reality, educated people in the ancient world overwhelmingly understood that the Earth was round. The Greeks not only believed this—they proved it. So let’s look at the evidence they had, because it’s not immediately obvious. When you look around, the world looks flat, right? So why would anyone think the Earth is curved?

The Stars

When we first looked at the stars, you’ll remember we noticed that one star—Polaris—didn’t seem to move at all. It looked like all the other stars were circling around it. The stars closest to Polaris stayed visible throughout the year, while those farther away dipped below the horizon at times.

Here’s a reminder of what that looks like:

Another thing to keep in mind: when you look at the stars, they all seem to move together, almost as if they were fixed on a giant sphere spinning around the Earth. If the Earth were flat, the sky would look the same no matter where you were. But something interesting happens when you travel north or south.

Let’s say you find Polaris in the sky, and then you walk north for several hundred miles. After a few days, you’ll notice something remarkable—Polaris is higher in the sky! Some of the bright stars near the horizon, like Capella, have shifted too—they’re higher than before.





How is that possible? Well, if both the Earth and the stars are on spherical surfaces, that’s exactly what we’d expect.

Here’s what that looks like zoomed out:




And here’s what it would look like from your perspective on Earth:



So why does this explain why Polaris appears higher?

In this diagram, the blue sphere is Earth, the orange dot is you standing on the surface, and the red dot is Polaris. The red line marks your horizon—your line of sight if you were looking straight ahead—and the purple line is what you’d see if you looked straight up.



As you can see, Polaris sits somewhere between those two lines. So you need to tilt your head upward a bit to see it. Now imagine walking north along the surface of the Earth. As you move north, you’d have to tilt your head higher and higher to see Polaris. If you went all the way to the North Pole, Polaris would appear directly overhead. And that’s exactly what we observe! The farther north you go, the higher Polaris climbs in the sky.



Likewise, if you travel south, Polaris gets lower and eventually disappears below the horizon. This change in the position of the stars depending on latitude is one of the first clues that the Earth is curved.

The Sailing Ships

Now let’s look at another classic observation. In ancient Greece, especially in port cities like Athens, people often watched ships sail away over the sea. If you stood by the harbor and watched, you’d notice something curious.

Check out this short video (watch for about a minute and a half):
https://www.youtube.com/watch?v=7nUFLLUahSI&t=23s

Here’s what happens:

At first, you see the whole ship clearly.



But as it sails farther away, the hull begins to disappear first—


—and soon only the mast is visible above the horizon:




Eventually, the entire ship disappears—not because it’s too small to see, but because it’s literally sinking out of view behind the curve of the Earth.

You can think of it like watching your friend walk toward you over a hill. You’d see their head appear first, then their shoulders, and finally their whole body as they come closer. The ship is doing the opposite—it’s sailing “down” the curve of the Earth, like going over the crest of a hill.

The Lunar Eclipse

There’s one more piece of striking evidence: the lunar eclipse.

Watch this video and see what stands out:
https://www.youtube.com/watch?v=buXTecdfqxo&start=15

What shape is the shadow crossing the Moon—and what do you think is casting it?

Here’s a snapshot: the shadow is curved, like the edge of a large circle.


The Greeks correctly reasoned that a lunar eclipse happens when the Earth passes between the Sun and the Moon, casting its shadow across the Moon’s surface. And because that shadow is always curved, they concluded that the Earth itself must be a sphere. No other shape would cast a perfectly circular shadow in every orientation.

Tying It Together

Each of these observations alone might not be enough to prove the Earth’s shape—but taken together, they form a powerful argument.

We often think of “proof” as something we can see directly. But this shows a deeper kind of reasoning: asking what would be different if the Earth were flat versus round, and then testing those predictions through observation. Even without ever leaving Earth, the Greeks were able to reason their way to the truth.

When you go outside, the ground looks flat. There are hills and valleys, but you don’t see a curve. Yet by gathering clues—from the stars, from ships at sea, and from the Moon’s shadow—the Greeks revealed that the world is far grander than it appears.

Next time, we’ll explore how the Greeks began thinking about motion—how things move on Earth and in the heavens.