Unabashed Aquinas fanboy here. I could prattle on and on about the triumph that is the Summa Theologiae, a massive work that is astonishing for its lucidity, erudition, coherence, and comprehensiveness, or about the sheer brilliance of the Summa Contra Gentiles, an amazing extended argument for the truth of the Christian Faith. These are the works he is most famous for, and that is not unjust.
But St. Thomas was not just a brilliant theologian and philosopher. He was also a great commentator. I have been privileged to read a few of his commentaries on the works of Aristotle, and they are extraordinarily great helps in understanding The Philosopher. These commentaries also include some gems of his own wisdom from time to time, and it is one of these that I would like to present for your enjoyment today.
Secondly, we must keep in mind that certain “anomalies,” i.e., irregularities, appear with respect to the motions of the planets. For the planets seem to be now swifter, now slower, now stationary, now retrogressing. Now this does not seem to be appropriate to heavenly motions, as is evident from what has been said above. Therefore, Plato first proposed this problem to an astronomer of his time, named Eudoxus, who tried to reduce these irregularities to a right order by assigning diverse motions to the planets; a project also undertaken by later astronomers in various ways. Yet it is not necessary that the various suppositions which they hit upon be true – for although these suppositions save the appearances, we are nevertheless not obliged to say that these suppositions are true, because perhaps there is some other way men have not yet grasped by which the things which appear as to the stars are saved. Aristotle nevertheless uses suppositions of this kind, in what regards the quality of the motions, as true (Commentary on de Caelo §451; emphasis added).
Where to start?
In the first place it is worth repeating that Aquinas and Aristotle (among others) knew that the Earth is a sphere. It is a fiction to say that Columbus held groundbreaking notions about the planet’s shape. There is, I suspect, not mere ignorance but also a certain conceit in the modern error about ancient opinion about this: as though we are so much smarter than those Bronze Age and medieval dunces. Wrong-o. Yes, they lacked technology, but their brains were plenty well practiced to figure this stuff out.
The second thing to notice is that they knew the motions of the planets were a problem for their astronomy. Aristotle did, as he wrote in De Caelo, but Aquinas goes farther. Where Aristotle was willing to accede to the opinions of the astronomers of his day, Aquinas was only willing to do so provisionally. In other words, he accepted the geocentric model advanced by astronomers but immediately points out that their theory is only one explanation of the movements of the planets. There may be others, he says, which might do the job as well or maybe even better.
And this is today’s reason why Aquinas is awesome. He correctly distinguished astronomy from both theology and revelation, and seems almost to guess that a better explanation than the Ptolemaic one might exist. In the interim, he accepts the results of astronomy as far as they go. St. Thomas was not dogmatic when the facts did not support dogma, and the fact is that astronomy is not theology. It is not revelation. There may be improvements in the theories astronomers would someday formulate, and St. Thomas was able to see and accept this fact.
Another point that might be worth making is that his view separates Aristotle’s philosophy from the contemporary astronomy of their respective times. Astronomy and philosophy are separate things, and to disprove the science of his day or of Aristotle’s is a completely different thing from disproving their philosophy. The latter still stands as a towering achievement.