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Comment by jan_Sate | Hacker News Reader

This estimation method would only work under the assumption that earth is a spherical. Now that we do know that it's a sphere. But how could you figure that out back in 2000 years ago?

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That’s the entire thing! He read an account of a city so many miles away from where he was that on a specific day of the year had no shadows at noon. On that day in his city, shadows still were cast. If there was no discrepancies in the shadows that would make sense for a flat earth. But the discrepancy made him determine it had to be sphere and then he went out and calculated the circumference.

Huh? If I put a lightbulb above something, the place it’s above won’t have shadows, but there will be shadows at places that aren’t directly underneath.

Only places near the equator will be parallel with the sun. Anywhere else, because the curve of the earth, there will always be an angle, and thus even at high noon on the summer solstice, there are still shadows in Alexandria. So I don’t understand your analogy. It doesn’t apply because the earth isn’t flat.

We know from Aristotle that they had some sense of how everything falling down would tend to produce a sphere, if "down" was perchance a point.

The Earth must be a sphere because during lunar eclipses its shadow is always a circle.

Because spheres are the best Platonic solid and so Earth must be a sphere, naturally.

Spheres aren't Platonic solids.

Whoops, I was thinking "solids that were highly significant to Plato" (for which spheres definitely qualify) but of course, as you point out, they're not actually Platonic solids. Still Plato relate-o though:

> Plato's Timaeus proposed that the body of the cosmos was made in the most perfect and uniform shape, that of a sphere containing the fixed stars.

(Wikipedia, referencing F. M. Cornford, Plato's Cosmology: The Timaeus of Plato)

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