How the circumference of the Earth and speed of light were first calculated

Contrary to the popular American belief that it was Columbus who discovered that the Earth was round, that fact was not only known a long time before, the circumference of the Earth had actually been calculated to high accuracy 2,200 years ago.

In this video clip, Adam Savage of Mythbusters explains how Eratosthenes calculated the Earth’s circumference using a simple idea, getting a figure that was within 1% of currently accepted values. He also explains how in 1849 Armand Fizeau got an accurate measurement for the speed of light within 2% of current values, long before sophisticated equipment came along.

(Via Machines Like Us.)


  1. Steve says

    The whole flat earth thing is a myth. By the middle ages it was common knowledge that the Earth is a sphere. Books and even religious paintings show as much.

    Especially for people who went to sea it was pretty obvious. At sea you can actually see the curvature of the Earth. And there are phenomena like seeing a mountain before seeing the beach when approaching land, or the mast of a ship before the rest.

  2. Steve says

    By the way, the accuracy of the Fizeau experiment was way worse than 2%. A year later he teamed up with Léon Foucault. They used a rotating mirror (where the angle indicated the speed of light) and achieved 1% accuracy

    And they weren’t even the first. James Bradley received the same results over a century earlier, although his discovery was accidental.

  3. 'Tis Himself says

    Contrary to popular belief, it was Columbus who was wrong, not the average educated European of the late 1400s. Columbus thought the circumference of the Earth was 25,255 km while the true figure was around 40,000 (as Eratosthenes determined). No ship in the 15th century could carry enough food and water for voyages of the real distance. Most Europeans concluded that undertaking a westward voyage from Europe to Asia would be impossible as crews would die of starvation or thirst before reaching Asia. That was the reason why Columbus had such trouble finding a sponsor for his proposed voyage to Asia.

    To his dying day, Columbus thought he had reached Asia.

  4. M Groesbeck says

    Will TED be eliminating this video, now? Last I heard, the stated policy was to censor any talk deemed “partisan”, by which they mean presentation of any facts which one or both of the dominant political parties in the U.S. might find inconvenient. The very existence of science is quite politically inconvenient; will TED be cutting out this vid?

  5. Guest fromHolland says

    Danish astronomer Ole Roemer beat Fizeau by almost 200 years (1676) by observing the Galilean moons of Jupiter. His estimate was some 25% off, but more important than the numerical value was the realisation that the speed of light was indeed finite.

  6. says

    Did Adam mess up his explanation? He says:

    “If you know the circumference of a circle and you have two points on it, all you need to know is the distance between those two points and you can extrapolate the circumference.”

    So…if you know a circle’s circumference, you can use that to calculate it’s circumference?

  7. Mano Singham says

    His explanation is correct but he misspoke. I think what he meant to say was something like “If you have a circle and you have two points on it…” or “If you know the Earth is a sphere and you have two points on it…”

  8. bryanfeir says

    To his dying day, Columbus thought he had reached Asia.

    Though, according to what I read in Daniel J. Boorstin’s ‘The Discoverers’, Columbus had at least some doubts about the distance numbers. Columbus wrote down two sets of distances sailed in his logs: both his own calculations based on speed, and the lies of shorter distances he told his crew because he didn’t want to have a mutiny if they reached the distance he had said at the start and the ships hadn’t seen land yet.

    Amusingly, given the best guesses as to where Columbus actually touched down first, Columbus apparently over-estimated on his speed calculations, and the lies for public consumption were actually closer to accurate than his private ‘real’ distance measurements.

  9. left0ver1under says

    For those who have never heard, both Adam Savage and Jamie Hyneman are both openly atheist.

  10. b9 says

    I doubt the common folk at the time of Columbus had any idea of wether the earth was round or flat. Today we have the best education system we’ve ever had, and we’re mass-schooling kids from a young age to adult-hood, and still half of americans doubt the theory of evolution and think the earth is 6000 years old, which is equally as ridiculous as thinking the earth is flat. And, 500 years from now, when the myth is going around that people where “dumb” enough to believe that the earth was only 6000 years old and evolution was false during the 21th century, that myth wouldn’t be entirely untrue.

  11. Julie says

    Actually, B9, Evolution is the myth. No proof. None. After all this time, only changes within species. It is probably the biggest lie in history! And, contrary to what we are told, MANY excellent, intelligent, learned scientists say that all of the evidence we have today points to SPECIAL CREATION. We can’t believe a lot of what our textbooks tells us – they do not teach the truth, they teach what the people in power want us to believe.

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