How did ancient people know that the earth was round?
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Look straight ahead of you. Can you see forever into the distance? No? Because the earth curves away.
Shadows. Shadows in one place on Earth are a certain length and direction, while at the same time in another place they’re different. The only explanation is a curved Earth.
The two major arguments known to the ancients were these:
* Observing that if you travel northwards or southwards by any specific distance, the angle of the North Star above the horizon will also change by a specific predictable amount, implying that the Earth’s surface has a consistent curvature, like a sphere.
* Observing that the shadow the Earth casts on the Moon during a lunar eclipse is always a circle, and realizing that the only shape which always casts a circular shadow is a sphere.
Most of it was determined the same way the sundial came into existence, object placed and using the sun to calculate positioning, determined that the shadow was not directly above, with movement being non-linear. Wasn’t even that long ago either in relation to history, maybe 500 BC? What made it more obvious was when lunar eclipse happened, and people were like…..AHA! If shadow round, we must be round!
Watch a ship sail out to sea. Once it get to the horizon, it starts to drop out of sight, until it’s completely gone. The simplest explanation is that the Earth is some sort of round.
Look at the shadow of the Earth on the Moon during a lunar eclipse. The Earth seems to have a circular silhouette — every time. Again, the simplest explanation is that the Earth is close to spherical.
Carl Sagan explained it best.
If you have nothing else to do than think about physics or how bad we were at it back then, you could reasonably conceive that we would deduce the theory. Perhaps not me, but an Einstein of the day.
So it all starts with the Greeks, though there is evidence it’s even further back from that. They looked up at the sky, particularly the moon, and said “That sure looks round to me… and so does the sun, while we’re at it… why WOULDN’T the Earth be?”
Along comes a smart guy we’ll call Eratosthenes because, well, that was his name. He learned that at midday, during the summer solstice in Syene, a stick stuck in the ground didn’t cast much of a shadow. So he asked himself “Eratosthenes, do you think this might be the case here in Alexandria where I am?” So he did the only sensible thing and stuck a stick in the ground at the same day and time. Lo and behold… there was a shadow. Well.. that’s not supposed to happen… So let’s look at that shadow… around 7 degrees or so. Interesting.
The Greeks had already figured out that however far away the sun was, it was a long freaking way, and shouldn’t have much effect on shadows of sticks. So if the shadow is 7 degrees, and I know how far it is from Syene to Alexandira, then I’ve got myself a sphere and I can work out how big it is with those numbers.
That’s exactly what he did, and he came up with a number that was less than 50 miles of what we now know to be the diameter of the Earth.
Maritime Navigators knew it first / have always known.
When rigging the sails, somebody securing the top mast at one point MUST have said “Ahoy! Enemy vessel to starboard side!”
Then that same person scrambled down the rigging only to find that the same ship was not visible from the deck.
Confused, this same person must have scrambled back up the top mast, only to see the enemy vessel slowly come into view from the top to the bottom. First the top mast appears. Then the sails. Then the prow. Then finally the hull.
What possible explanation could they have derived other than “the ocean seems to run downhill. Always. In every direction. So: the earth must curve downhill. Always. In every direction.”
That’s why pirate ships have the lookout stationed in the “Crow’s Nest.” You’ll see the enemy sooner from up high. The only explanation? The earth is curved.
Among the easiest ways to detect that the Earth is round is that a mountain, ship, or building will slowly disappear below the horizon as you move away or go to a lower elevation, or appear in the reverse. The only way that makes sense is if you are on a round surface.
Eratosthenes (276-194 BC) realized you could measure the circumference by measuring the length of a shadow of objects of known height at noon from two points at known differing latitudes. He got a result of 252,000 stadia or about 40,000 km, which is within 2.5%.
The idea that the earth is flat is kind of weird if you think about it. The earth is self evidently round. The horizon alone is proof of it. On top of that even the simplest of maths and scientific method (such as practiced by Archimedes and Pythagoras) makes it very very obvious. The only people who have ever believed the earth was flat were the wilfully ignorant.
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