3 And let no one suppose it to be a thing incredible that the brightness of the light is one thing, and the body which is its material vehicle is another. First, in all composite things, we distinguish substance susceptible of quality, and the quality which it receives. The nature of whiteness is one thing, another is that of the body which is whitened; thus the natures differ which we have just seen reunited by the power of the Creator. And do not tell me that it is impossible to separate them. Even I do not pretend to be able to separate light from the body of the sun; but I maintain that that which we separate in thought, may be separated in reality by the Creator of nature. You cannot, moreover, separate the brightness of fire from the virtue of burning which it possesses; but God, who wished to attract His servant by a wonderful sight, set a fire in the burning bush, which displayed all the brilliancy of flame while its devouring property was dormant. It is that which the Psalmist affirms in saying “The voice of the Lord divides the flames of fire.” Thus, in the requital which awaits us after this life, a mysterious voice seems to tell us that the double nature of fire will be divided; the just will enjoy its light, and the torment of its heat will be the torture of the wicked.
In the revolutions of the moon we find anew proof of what we have advanced. When it stops and grows less it does not consume itself in all its body, but in the measure that it deposits or absorbs the light which surrounds it, it presents to us the image of its decrease or of its increase. If we wish an evident proof that the moon does not consume its body when at rest, we have only to open our eyes. If you look at it in a cloudless and clear sky, you observe, when it has taken the complete form of a crescent, that the part, which is dark and not lighted up, describes a circle equal to that which the full moon forms. Thus the eye can take in the whole circle, if it adds to the illuminated part this obscure and dark curve. And do not tell me that the light of the moon is borrowed, diminishing or increasing in proportion as it approaches or recedes from the sun. That is not now the object of our research; we only wish to prove that its body differs from the light which makes it shine. I wish you to have the same idea of the sun; except however that the one, after having once received light and having mixed it with its substance, does not lay it down again, while the other, turn by turn, putting off and reclothing itself again with light, proves by that which takes place in itself what we have said of the sun.
The sun and moon thus received the command to divide the day from the night. God had already separated light from darkness; then He placed their natures in opposition, so that they could not mingle, and that there could never be anything in common between darkness and light. You see what a shadow is during the day; that is precisely the nature of darkness during the night. If, at the appearance of a light, the shadow always falls on the opposite side; if in the morning it extends towards the setting sun; if in the evening it inclines towards the rising sun, and at mid-day turns towards the north; night retires into the regions opposed to the rays of the sun, since it is by nature only the shadow of the earth. Because, in the same way that, during the day, shadow is produced by a body which intercepts the light, night comes naturally when the air which surrounds the earth is in shadow. And this is precisely what Scripture says, “God divided the light from the darkness.” Thus darkness fled at the approach of light, the two being at their first creation divided by a natural antipathy. Now God commanded the sun to measure the day, and the moon, whenever she rounds her disc, to rule the night. For then these two luminaries are almost diametrically opposed; when the sun rises, the full moon disappears from the horizon, to re-appear in the east at the moment the sun sets. It matters little to our subject if in other phases the light of the moon does not correspond exactly with night. It is none the less true, that when at its perfection it makes the stars to turn pale and lightens up the earth with the splendour of its light, it reigns over the night, and in concert with the sun divides the duration of it in equal parts.
Source: Nine Homilies of Hexaemeron (New Advent)