• Question: How did we find out how the earth was round in the first place?

    Asked by lazamonsta to Dilwar, Lou, Rachel, Simon, Susan on 11 Nov 2013.
    • Photo: Simon Langley-Evans

      Simon Langley-Evans answered on 11 Nov 2013:

      Personally I don’t believe that any body ever believed it was flat. Anyone who stands on a shore line or on a hilltop can see the curve of the earth very clearly. If you watch a ship heading out to sea, if dips below the horizon and that clearly shows that the earth is curved. Also if you look at the moon as it goes through its phases you can not only see that it is curved, you can also see that the shadow of the earth during an eclipse is curved.

      The first thinker who proved the Earth is round was a Greek called Eratosthenes. He did some experiments measuring the shadow cast by a stick in the noon sun but at different locations. From the answers he was able to show that the Earth was curved. This was in 240 BC.

    • Photo: Susan Skelton

      Susan Skelton answered on 12 Nov 2013:

      A Greek called Eratosthenes worked out that Earth is round in 240BC.

      He started thinking about this problem when he was told something really amazing from a friend in the city of Syene in southern Egypt. He was told that on the Summer Solstice, the shadow of someone looking down a deep well would completely block the reflection of the Sun at noon.

      In other words, the Sun would be directly overhead at this time, not a single degree to the South, North, East or West. And if you had a completely vertical object, it would cast absolutely no shadow.

      But Eratosthenes knew that this wasn’t the case where he was, in the city of Alexandria. Vertical objects always cast shadows.

      So Eratosthenes did the experiment. He measured the length of the shadow cast by a vertical stick at noon, and figured out what angle the Sun made with the vertical direction at Alexandria.

      And he calculated this to be 7.2 degrees. But at this time, in Syene, the angle the Sun was making with an identical vertical stick was zero degrees!

      What could be causing this?

      Eratosthenes realised that the Sun’s rays could all be parallel, and that the Earth could be curved!

      By measuring the distance between the two cities (which he did on a camel) he could calculate the circumference of the world.

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