Wnderer wrote: 1001usernames wrote:
orthogon wrote:What did they think about gravity at the time? Was it just natural to assume it would be normal to the Earth's surface everywhere, or did they think it might act in the same direction everywhere and that you might fall off when you got to the "underside"? (I guess they'd been to India where gravity should have been nearer to the horizontal than the vertical, so they had a pretty good idea.)
Yeah, I know the general principle of how it works was known to be true at least from the time of Dante, and almost certainly from the Greek era; if you read the "Inferno," he actually has Dante and Virgil switch directions when they are climbing down the lowest pit of Hell (along Satan's legs, actually!). I'm willing to bet that Dante, being the Aristotelian that he is, got it from Aristotle, though I don't know that for a fact. The physics of it is a little off (he assumes that there's a sort of instant switch, instead of gradually decreasing gravity, then gradually increasing in the other direction), but the knowledge is still there.
Aristotelian Gravity worked like a type of buoyancy. The universe was geocentric and all five of the elements wanted to organize themselves into layers, so that earth was at the center, water was in a shell above earth, air was in a shell above water, fire was in a shell above air and aether was in a shell above fire. They knew the earth was round but they also thought the earth was the center of the universe.
To be fair, not *all* ancient Greek scientists thought that Earth was the center of the universe. While his measurements were off by a factor of 20 due to an error of a couple degrees in a measurement, Aristarchus correctly came to the conclusion that the sun is much further away than the moon, and since they take up approximately the same size in the sky, much larger - than not just the moon, but than the Earth as well. He thus proposed that the Earth orbits around the sun. His texts doing so have not survived, but mention of them has. The obvious criticism was that if the Earth revolved around the sun, then one should see parallax in the stars (obviously, his response was that the stars were way far away, but in the time of the Greeks, such distances probably seemed absurd to many astronomers, leading to it generally being seen more as a rival theory than the predominant one).
Archimedes once used Aristarchus's work to estimate how many grains of sand it would take to fill the universe - and, by more important side-calculation, the diameter of the universe. He makes a weird assumption that the day-night solar parallax is the same as the annual stellar parallax, and also contains Aristarchus's measurement errors, yet comes to the conclusion that the universe is what we'd today call two light years in diameter.
Aetios says that Aristarchus "sets the sun among the fixed stars and holds that the earth moves around the elliptic", which would seem to imply that he thought the stars were just other suns, and thus it would be natural to place them much further away for them to appear so dim.
Based on his figures, Aristarchus apparently estimated the human limit of angular discrimination at about 0.0001 radians. If this is correct, then it would imply that in Aristarchos's universe the stars were at least on the order of tens of thousands of AU away (1 light year ~= 63.000 AU).
It's such a shame that nothing directly from Aristarchus has survived, because the mentions of his work really are fascinating. And clearly he was known in the ancient world. And there were other scientists who followed up on his work. Poseidonius, for example, got a much more accurate measurement of the distance from the earth to the sun (and thus size of the sun), only off by half - and suggested that other stars can even exceed the sun in size. Seleucus (whose works were also lost) also reportedly argued for a heliocentric universe, and more than that, an infinitely large one (his reasoning for heliocentrism was not based on astronomical measurements, but is believed to have been based on the tides). And on and on - so we know Aristarchus was hardly alone. Heck, given that we know that this was being talked about for centuries on end, and the figures refined and such, and that it was proposed that the sun itself was just another star - I wouldn't be shocked to learn of the ancient Greeks had gotten past heliocentrism itself.