11 On its variations depends also the condition of the air, as is proved by sudden disturbances which often come after the new moon, in the midst of a calm and of a stillness in the winds, to agitate the clouds and to hurl them against each other; as the flux and reflux in straits, and the ebb and flow of the ocean prove, so that those who live on its shores see it regularly following the revolutions of the moon. The waters of straits approach and retreat from one shore to the other during the different phases of the moon; but, when she is new, they have not an instant of rest, and move in perpetual swaying to and fro, until the moon, reappearing, regulates their reflux. As to the Western sea, we see it in its ebb and flow now return into its bed, and now overflow, as the moon draws it back by her respiration and then, by her expiration, urges it to its own boundaries.
I have entered into these details, to show you the grandeur of the luminaries, and to make you see that, in the inspired words, there is not one idle syllable. And yet my sermon has scarcely touched on any important point; there are many other discoveries about the size and distance of the sun and moon to which any one who will make a serious study of their action and of their characteristics may arrive by the aid of reason. Let me then ingenuously make an avowal of my weakness, for fear that you should measure the mighty works of the Creator by my words. The little that I have said ought the rather to make you conjecture the marvels on which I have omitted to dwell. We must not then measure the moon with the eye, but with the reason. Reason, for the discovery of truth, is much surer than the eye.
Everywhere ridiculous old women's tales, imagined in the delirium of drunkenness, have been circulated; such as that enchantments can remove the moon from its place and make it descend to the earth. How could a magician's charm shake that of which the Most High has laid the foundations? And if once torn out what place could hold it?
Do you wish from slight indications to have a proof of the moon's size? All the towns in the world, however distant from each other, equally receive the light from the moon in those streets that are turned towards its rising. If she did not look on all face to face, those only would be entirely lighted up which were exactly opposite; as to those beyond the extremities of her disc, they would only receive diverted and oblique rays. It is this effect which the light of lamps produces in houses; if a lamp is surrounded by several persons, only the shadow of the person who is directly opposite to it is cast in a straight line, the others follow inclined lines on each side. In the same way, if the body of the moon were not of an immense and prodigious size she could not extend herself alike to all. In reality, when the moon rises in the equinoctial regions, all equally enjoy her light, both those who inhabit the icy zone, under the revolutions of the Bear, and those who dwell in the extreme south in the neighbourhood of the torrid zone. She gives us an idea of her size by appearing to be face to face with all people. Who then can deny the immensity of a body which divides itself equally over such a wide extent?
But enough on the greatness of the sun and moon. May He Who has given us intelligence to recognise in the smallest objects of creation the great wisdom of the Contriver make us find in great bodies a still higher idea of their Creator. However, compared with their Author, the sun and moon are but a fly and an ant. The whole universe cannot give us a right idea of the greatness of God; and it is only by signs, weak and slight in themselves, often by the help of the smallest insects and of the least plants, that we raise ourselves to Him. Content with these words let us offer our thanks, I to Him who has given me the ministry of the Word, you to Him who feeds you with spiritual food; Who, even at this moment, makes you find in my weak voice the strength of barley bread. May He feed you for ever, and in proportion to your faith grant you the manifestation of the Spirit in Jesus Christ our Lord, to whom be glory and power for ever and ever. Amen.
Source: Nine Homilies of Hexaemeron (New Advent)