The word of the day is Geodesy, and it has an awesome animation to explain it

[andrewducker]

Comments

[artkouros.livejournal.com]

I've always wondered how the Egyptians did that measurement trick. How the the guy in the second city know that the sun was exactly overhead in the first city?

[fearmeforiampink]

IIRC, it involves looking down a well or somesuch; a long tube where the sunlight would only get if the sun was directly above.

[andrewducker]

Yes, but how does the person in city 2 know that the sun is directly overhead in city 1?

[spacelem.livejournal.com]

You can do it if you can determine that city 2 is precisely north of the well in city 1 (which you could do if you had a compass). If it is north, then 12 pm (or whatever you call the time when the sun is directly overhead, presumably they didn't arse about with daylight savings back then) occurs at both sites simultaneously. You can determine 12 pm by observing shadows cast by an upright stick (which you can do with a plumb line).

[andrewducker]

Well, it's not going to be _directly_ overhead in either, as the equator is South of both of them. So it'll have to be the moment when it's at its overheadiest.

[apostle-of-eris.livejournal.com]

Eratosthenes was indeed made of awesome, but the Egyptians get serious credit too. The famous well is exactly on the Tropic of Cancer. On midsummer's day, only, the sun shines down it. The placement was totally deliberate; it even may be why the temple it was inside of was there. (My memory hasn't been refreshed for a while.)
So once the well's special placement/property was known, and it had never been a secret, someone only had to check the angle of the noon sun in Alexandria on midsummer's day and know the distance to Syene.
Iirc, Eratosthenes was less than 5% off.