Tosfos D"H KeKadur quotes the Yerushalmi on the origin of the knowledge that the earth is spherical.
The Yerushalmi Avodah Zarah 3:1 (p. 18b) relates that Alexander the Macedonian soared high enough to see the earth as a sphere.
The Pnei Moshe commentary adds that Alexander accomplished the high altitude via an eagle-back 'ride'.
There is a great difficulty with this elucidation.
The curvature of the earth cannot be observed from the cruising altitude of commercial airliners, i.e. approx. 36000 feet, yet cabin pressurization is required to enable breathing, the outside air being too rarified of oxygen.
How then was Alexander able to reach an altitude high enough to see the curvature of the earth? It is highly unlikely that his contemporaries had the technology to produce pressurized oxygen tanks.
Furthermore, the maximum altitude reachable by an eagle, even unburdened by a person's weight, is considerably inferior to the cruising altitude of commercial jetliners.
I take the Yerushalmi to mean not that Alexander actually saw the curved limb of the earth, but that he was high enough to discern that the earth was a sphere. I don't think it would be difficult to see proof of the spherical shape of the earth from atop a gliding eagle - or even from a simple jet airliner, for that matter.
1. The earth has a horizon, meaning that the surface that one is observing from is not an infinite plane. On the clearest of days, the only restriction to one's range of sight is the horizon. There can be two explanations for this - one, that the Earth at some point just stops, as if you were looking off the edge of a table. The other is that the Earth is round. If one rises above the earth and the view to the horizon gets increasingly greater, then, the second choice must be correct.
2. In a related context, next time you are on a ferry or other ship and still in sight of land note the view of land from as low as you can get on the ship, and then go to the highest deck you are allowed on and repeat. Comparing the views shows clearly that from the higher view, more of the land can be seen (i.e. the lower part of the land-mass becomes visible). This only makes sense on a curved earth, in which the lower part of the land mass 'sinks' beneath the horizon as we distance ourselves from it. This effect would be especially noticeable during takeoff - whether on back of an eagle or a jetliner. (For graphic detail, see here)
3. And if you find it hard to believe that eagles once were so much hardier than they are now - take the Yerushalmi's account as a dream that Alexander experienced, in which he was shown the earth from above in the manner described.
The proof of the earth's geometry, then, would not be from the dream itself (Divrei Chalomos Lo Ma'alin...), but from the fact that even the Greeks of Alexander's time must have assumed the earth to be round (which, by the way, is true), and that is why he described his view in the terms that he did. And that is why the depicted the earth in the hands of their idols as a globe.
Kollel Iyun Hadaf