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Asked by 652spaa38 to Heather, Helen, Hugh, Jane, Julian on 18 Nov 2014.
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Julian Onions answered on 18 Nov 2014:
It was the ancient Greeks who first worked this out.
Eratosthenes managed to show the Earth was round and even measured about how round the Earth was in around 100BC
He did it by looking at shadows cast, with sticks and that the Sun was overhead shown by it beaming directly down a well in Syrene.
Some simple maths and he could work out approximately how round the Earth was.
Another clue came from lunar eclipses, where the earth casts a shadow on the moon. You can see from this that it is faintly curved shadow cast, so but be from a circular object.
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Hugh Osborn answered on 18 Nov 2014:
While the ancient Greeks were the first to measure the roundness of the Earth (Eratosthenes managed to get it to within 2% more than 2000 years ago), there’s no reason to think ithe idea was new. In fact, any ardent observer of the night sky might come to the conclusion that the Earth was round.
The moon, with it’s waxing and waning crescents, can easily be identified as a sphere. It’s morion and eclipses also suggest it is moving round us in an orderly way. The fact that the sun gets lower as you move North or South from the equator is also perplexing if the Earth is round. And, as Julian mentioned, lunar eclipses also imply a spherical Earth.
So we don’t really know when people started to realise that the Earth was round: we just know that as far back as 100BC humans had known and tested that fact.
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Helen Johnson answered on 19 Nov 2014:
Agreed. I reckon the ancient Greeks and Romans knew a lot more than we sometimes give them credit for 😉
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