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Book II Lecture 27
http://dhspriory.org/thomas/english/DeCoelo.htm#2-27

St. Thomas says that Aristotle determined the truth about the shape of the earth and proved that it is spherical.

St. Thomas does not simply describe Aristotle's opinion, but call it the truth.

Yes, he does.  As for Aristotle's argument however, it doesn't address many other things we now know to be true.  It certainly isn't drawn from Scripture, nor considered in light of revelation.  His description of shadows is provably wrong.  And really, what do either of them know about the weight of the earth or how the land gathered up in a balanced way to form a ball? His argument is done in a since this/then that way leaving a lot of scientific proof to be desired.  St. Thomas believes him, but Aristotle's proof is seriously lacking.  Trusting a pagan, no matter how smart he sounds, is a risky business.  Can't come down on him too hard.  I believed them once, too.            
   

My point is, not the science is correct, but that St. Thomas saw this as a subject in which one could determine the truth by using science.  In order for him to think that, he must have understood that Scripture is silent on the shape of the earth.  If he interpreted Scripture as you do, he would have objected to Aristotle's ideas.

One of the main purposes of these commentaries on Aristotle was for St. Thomas to consider the philosopher's work in the light of Revelation.  When St. Thomas thinks Aristotle's ideas are not compatible with Christianity, St. Thomas says so.  So his comments here not only show him teaching spherical earth is the truth, but also teaching that spherical earth is compatible with the Catholic faith.

St. Thomas does say one could determine truth by other means, like science, (as we discussed before) but that doesn't mean Scripture is silent about the shape of the earth.  St. Thomas was a brilliant guy, no question, but everyone can't be brilliant about every thing.  Not knowing more about this isn't a crime.  Had he been familiar with it more, he would have said, "Some say Scripture says..." or "According to Scripture..." or "Scripture says this, describes that, so I say..."  He didn't do that, even though several other Saints so enthusiastically did.  Scripture's strict adherence to the picture it paints is a huge elephant in the room, but St. Thomas never uses it for the sphere, which is telling. He does seem enthralled with Aristotle and played his mind candy approach to figuring, but most comes across as mentor applause.  He certainly respected Aristotle. This isn't necessarily proof for either view, honestly.  Seems that teaching spherical earth is compatible with the Catholic Church would take positive information, from somewhere. Absence of information falls short.  As for St. Thomas' conclusion, I whisper respectfully, "that's his opinion."  

St. Bede, St. Thomas, and St. Albert (all Doctors of the Church) all discussed the shape of the earth in terms of science.  They, along with virtually every other educated Catholic throughout the middle ages, all interpreted Scripture as leaving the matter open to be determined by science.  

Before them, St. Augustine, St. Basil, and St. John Damascene had all taught that the shape of the earth was not a matter of faith and unimportant to it. (And the quotes have all appeared on this forum already so don't ask me for them again.) They all took the position that teaching spherical earth is compatible with Catholicism.  This was accepted so that it was no longer an issue by the middle ages.

The few Saints during the patristic period who actually taught the earth was flat were associated with the Antiochian approach to Scripture interpretation which did not persist in Catholic thinking.  Their opinion about flat earth disappeared along with it.

I agree, the Saints all discussed the shape of the earth in terms of science.  And some Saints did not think it was important.  Also agreed. But your last statement is only partially true.  Many slipped away from their concern for the subject, but that doesn't mean it doesn't matter.  Our conversation here, and that it appears to be tied to the apostasy and the nєω ωσrℓ∂ σr∂єr suggests there is more to know about this that does matter.