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"...the Greek philosopher Eratosthenes noted that the sun cast a shadow of different lengths at different latitudes (all the shadows would be the same length if
by 13of40 11y ago
"...the Greek philosopher Eratosthenes noted that the sun cast a shadow of different lengths at different latitudes (all the shadows would be the same length if the earth's surface were flat)."
This is an interesting statement, because it means he had some idea of how far away the sun was from the Earth. If it was, say, a thousand miles up, you'd see huge differences in shadow length depending on where you were standing on a flat earth. Since it's tens of millions of miles away, the difference is negligible.
- pdonis 11y ago> it means he had some idea of how far away the sun was from the Earth He didn't really have any idea of the actual distance to the Sun. He just knew it was far enough away that the Sun's rays could be assumed to be parallel.
- bladedtoys 11y agoEratosthenes did not know the sun-earth distance and tentatively assumed it was infinite for that measurement. But then his successor Aristarchus under took to measure the sun-earth distance as follows: He looked at the angle the earth and moon form when the moon is exactly half full. This would give the sun distance as a multiple of the moon's distance using trigonometry. He observed the earth's shadow during lunar eclipses. This gave the moon's size in terms of earth size. He knew the earth's size from measuring shadows at different latitudes on earth and the distance between those sites. Knowing the moons size vs apparent size, simple trigonometry gave him the distance to the moon. And therefor also the distance to the sun. Unfortunately his angle measuring techniques were primitive and he decided the sun was 20x further from earth than the moon (rather than about 400x). But his other figures are quite passable.
- angersock 11y agoIf he was only off by an order of magnitude, given the equipment at the time, that's still quite impressive!