Thanks, Jayne. This is the quote. It says a lot. St. Thomas is clear, that if the earth is stable, it must be flat.
Concludit propositum: quod terra sit in medio mundi quia omnia corpora gravia moventur ad medium terræ. (That the earth is in the middle of the universe and all heavy bodies are moved per se to the middle of the earth ) Et sic, ex præmissis, nihil movetur in loco ad quem naturaliter movetur, quia ibi naturaliter quiescit. Sed terra aliquando movetur ad medium mundi, (from the foregoing as follows: Nothing is moved in the place toward which it is naturally moved. But the earth is naturally moved to the middle of the world.) ut probatum est, ergo, terra nullo modo movetur. (Therefore the earth is not in motion in any way)
After all these commentaries, he concluded that to be stable, the earth must be flat: Necesse est terram, ad hoc quod quiescat, habere figuram latam:( that if the earth is to be at rest, it has to be flat.) nam figura sphærica facile mobilis est quia in modico tangit superficiem, sed figura lata secundum se totam tangit superficiem, et ideo est apta ad quietem. (For a spherical shape is easy to move, because so little of it is in contact with a plane; but a wide shape is totally in contact with a plane, and is consequently apt for rest and to be firm.)
I am interested in an english translation of the whole context of this part. Also, I'm interested with an accurate english translation of the whole Summa, if you know of a particular edition for me to look for.
What you quoted makes sense, but dare I say St Thomas appears to not cover his bases here and so may be in error, unless the context around these quotes prove otherwise.
Having only read a few of his chapters, or whatever they are called, as posted on CI either in their entirety or close to it, I have found it confusing at times distinguishing his conclusions from opposing arguments. Given that me, a fallible human, can make this simple mistake, perhaps you did too, and quoted a false argument that St Thomas intends to prove wrong.