[mabbo]

As I've read it, he got a very close estimate, but only because he made two errors that happened to cancel each other out.

As well, he didn't actually measure the shadows himself, but heard stories about there being no shadow at a specific city.

[bjoli]

What he did was simple geometry, and in a way he was the right man in the right place. He heard that the sun was in zenith during the summer solstice ina certain town. He had a good runner measure the distance between Alexandria and that town and then measured the angle of the sun during the solistice in Alexandria (for simplicity, say it was 6 degrees). To get the full circle, just multiple by 60.

6th graders maths.

[paradoxparalax]

Is a pity that we have lost "his" library by human stupidity. There could have been a lot more to know about the state-of-art of astrography at his time.