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.