How did Eratosthenes tell the time?

[Pinotage]

In the Guardian* it said Eratosthenes calculated the size of the Earth by comparing shadows cast at midday in two points 800km apart. They didn't have clocks in 240BC. How did the two observers know it was the same time?

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*http://www.guardian.co.uk/science/story/0,3605,790
246,00.html

[geofbob]

Wherever you are, a shadow is at its shortest at noon. So Eratosthenes only had to compare the shadows at their shortest. In fact, most versions of the story state that Eratosthenes, who lived in Alexandria, learned that on the Summer solstice there was NO noon shadow at Aswan - because the sun is directly overhead - so he only had to measure the shadow at noon (ie the shortest shadow) on the same day in his home town, in order to do the calculation.