9 “And God made two great lights.” The word “great,” if, for example we say it of the heaven of the earth or of the sea, may have an absolute sense; but ordinarily it has only a relative meaning, as a great horse, or a great ox. It is not that these animals are of an immoderate size, but that in comparison with their like they deserve the title of great. What idea shall we ourselves form here of greatness? Shall it be the idea that we have of it in the ant and in all the little creatures of nature, which we call great in comparison with those like themselves, and to show their superiority over them?
Or shall we predicate greatness of the luminaries, as of the natural greatness inherent in them? As for me, I think so. If the sun and moon are great, it is not in comparison with the smaller stars, but because they have such a circumference that the splendour which they diffuse lights up the heavens and the air, embracing at the same time earth and sea. In whatever part of heaven they may be, whether rising, or setting, or in mid heaven, they appear always the same in the eyes of men, a manifest proof of their prodigious size.
For the whole extent of heaven cannot make them appear greater in one place and smaller in another. Objects which we see afar off appear dwarfed to our eyes, and in measure as they approach us we can form a juster idea of their size. But there is no one who can be nearer or more distant from the sun. All the inhabitants of the earth see it at the same distance. Indians and Britons see it of the same size. The people of the East do not see it decrease in magnitude when it sets; those of the West do not find it smaller when it rises.
If it is in the middle of the heavens it does not vary in either aspect. Do not be deceived by mere appearance, and because it looks a cubit's breadth, imagine it to be no bigger. At a very great distance objects always lose size in our eyes; sight, not being able to clear the intermediary space, is as it were exhausted in the middle of its course, and only a small part of it reaches the visible object. Our power of sight is small and makes all we see seem small, affecting what it sees by its own condition.
Thus, then, if sight is mistaken its testimony is fallible. Recall your own impressions and you will find in yourself the proof of my words. If you have ever from the top of a high mountain looked at a large and level plain, how big did the yokes of oxen appear to you? How big were the ploughmen themselves? Did they not look like ants? If from the top of a commanding rock, looking over the wide sea, you cast your eyes over the vast extent how big did the greatest islands appear to you?
How large did one of those barks of great tonnage, which unfurl their white sails to the blue sea, appear to you. Did it not look smaller than a dove? It is because sight, as I have just told you, loses itself in the air, becomes weak and cannot seize with exactness the object which it sees. And further: your sight shows you high mountains intersected by valleys as rounded and smooth, because it reaches only to the salient parts, and is not able, on account of its weakness, to penetrate into the valleys which separate them.
It does not even preserve the form of objects, and thinks that all square towers are round. Thus all proves that at a great distance sight only presents to us obscure and confused objects. The luminary is then great, according to the witness of Scripture, and infinitely greater than it appears.
Source: Nine Homilies of Hexaemeron (New Advent)