St. Thomas demonstrates that the (apparently) prevailing scientific judgment of his day was that the earth was round, not flat (of course, that doesn't mean that the Ptolemaic system was generally correct, obviously):
Sciences are differentiated according to the various means through which knowledge is obtained. For the astronomer and the physicist both may prove the same conclusion: that the earth, for instance, is round: the astronomer by means of mathematics (i.e. abstracting from matter), but the physicist by means of matter itself (ST Q1 A1 ad 2).And also:
Those who say that paradise was on the equinoctial line are of opinion that such a situation is most temperate, on account of the unvarying equality of day and night; that it is never too cold there, because the sun is never too far off; and never too hot, because, although the sun passes over the heads of the inhabitants, it does not remain long in that position. However, Aristotle distinctly says (Meteor. ii, 5) that such a region is uninhabitable on account of the heat. This seems to be more probable; because, even those regions where the sun does not pass vertically overhead, are extremely hot on account of the mere proximity of the sun. But whatever be the truth of the matter, we must hold that paradise was situated in a most temperate situation, whether on the equator or elsewhere (ST Q102 A2 ad 4).