To be fair, the Greeks assumed the Earth was round before they had very good evidence. The philosopher and mathematician Pythagoras is credited as the first to propose a spherical Earth back in 500 B.C., though he did so on aesthetic grounds: He believed the sphere was the most perfect shape. (We should note that Pythagoras was probably just the first Western thinker to propose this — it's reasonable to think that seafaring societies like those of the ancient Polynesians likely had some idea before that). A century later, the philosopher Plato suggested the same thing, which boosted the idea's popularity.

But when it comes to proving the Earth is round, Aristotle was the first Greek philosopher to put his money where his mouth was. When writing his book "On the Heavens" in 350 B.C., he laid out several pieces of evidence for the Earth having a spherical shape. For one thing, he pointed out, you can see the shadow of the Earth on the moon during a lunar eclipse — and since that shadow is always round regardless of where the Earth is in its rotation, you know the Earth is round.

For another thing, the stars are in different positions depending on where you are on Earth: There were stars in Egypt that couldn't be seen in Cyprus 600 miles (1,000 kilometers) away. That proved, he wrote, "not only that the earth is circular in shape, but also that it is a sphere of no great size: for otherwise the effect of so slight a change of place would not be quickly apparent."

Later, another scholar did Aristotle one better: He didn't just prove the Earth was a sphere; he actually measured its circumference. And he did it with only the sun and a stick.

Eratosthenes was one of the most prominent scholars of his time, dabbling in almost all of the sciences. In 240 B.C., he was appointed chief librarian of the library of Alexandria — a universal reference center unparalleled in size that was pretty much the Google of its day. One of his many ambitions was to make a map of the entire world, and to do that, Eratosthenes knew he'd need to determine the size of the Earth.

He had heard stories about a well in the city of Syene that had a peculiar characteristic. At noon on the summer solstice when the sun is directly overhead, sunlight illuminated the entire bottom of the well without casting any shadows. To check if the same thing would happen in Alexandria, Eratosthenes placed a stick upright in the ground at noon on the summer solstice. The sun did cast a shadow, at an angle of 7.2 degrees.

Eratosthenes realized that he could just do some simple calculations with that measurement and easily find the circumference of the Earth; he'd just need to know the distance between Syene and Alexandria. Distance measurements were rough at the time, but he hired some bematists — professional walkers, basically, who could precisely measure their steps — and found that the distance between the cities was about 5,000 stadia. That's roughly between 500 and 600 miles (800 and 900 kilometers), depending on which version of that unit of measurement scholars think he used.

With that, he could do a quick back-of-the-scroll calculation. Since the sun hit straight-on in Syene and at a 7.2-degree angle in Alexandria, the distance between them should be a 7.2-degree chunk of the 360-degree sphere that is the Earth. He also knew that that 7.2-degree chunk was roughly 500 miles long. It turns out that 7.2 is to 360 what 500 is to 25,000 — and that was his answer. The Earth was 25,000 miles (40,000 kilometers) around.

So, was Eratosthenes right? Even though some of his assumptions were faulty and his distances were rough, he was only off by about 100 miles. Today, we know that the Earth is 24,901 miles (40,075 kilometers) around at its equator, a little bit less if you measure pole-to-pole. Not bad for a guy and his stick.

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Read the whole story in "Circumference: Eratosthenes and the Ancient Quest to Measure the Globe" by Nicholas Nicastro. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.