If you’ve ever flown across the world, you’ll know one thing for sure: it’s a very, very long way.

It’s not the biggest planet in our solar system by a long way, but in terms of the perspective of a human being, our Earth is absolutely huge.

Of course, ‘huge’ is not a very specific measurement. If you want to know precisely, Earth has a circumference of approximately 40,075 kilometers at the equator.

How do we know that? Well, we measured it, of course.

By ‘we’, of course that means the royal ‘we’, or humankind as a species.

Nowadays we can use GPS and satellites to measure the Earth with some precision – but our measurement of the Earth has been known for much longer than we’ve had satellites orbiting our planet.

In fact, we’ve had some knowledge of the circumference of the Earth since 240 BCE, when a guy named Eratosthenes – who suspected that our planet was, indeed, spherical – decided to measure it using the sun, as a Union University article explains:

“At the summer solstice, shafts of sunlight illuminate the bottoms of deep wells in Syene. Eratosthenes realized the Sun had to be directly overhead Syene on that day. He also knew the Sun was 7 degrees away from the zenith at Alexandria on the solstice (which happens to be about 1/50th of the circumference of a circle). Using simple geometry, Eratosthenes then showed that this implied that the distance from Alexandria to Syene was 1/50th of the Earth’s circumference.”

Now, Eratosthenes understood how far Alexandria was from Syene (the city now known as Aswan), he was able to calculate the different angles of shadow when the sun hit a stick in both Alexandria and Syene, and his knowledge of how long it took a camel to travel between the two cities.

This early geometry allowed Eratosthenes to come up with his calculation for the circumference of the Earth, as the article continues:

“Using the rate that the average camel could travel, and the number of days that it took a camel to travel from Alexandria to Syene, he estimated that the distance from Syene to Alexandria was 5000 stadia. Since he had calculated this distance to also be 1/50th of the Earth’s circumference, then 50 times 5000 stadia yields 250,000 stadia for the Earth’s circumference. Now the stadium (singular of stadia) had different lengths in ancient times. Traditionally, it is assumed that there are 6 stadia in a kilometer (km), meaning that Eratosthenes found the Earth’s circumference to be about 42,000 km.”

Startlingly, he wasn’t far off – his calculation was only four per cent larger than the actual measurement – and this was done all without technology, just with the power of geometry and Eratosthenes astute attention to detail.

Impressive, right?

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