V. The sun and moon were created for use, not for worship
This objectionable practice must be given up therefore by the faithful, and the honour due to God alone must not be mixed up with those men's rites who serve their fellow-creatures. For the divine Scripture says: “You shall worship the Lord your God, and Him only shall you serve.” And the blessed Job, “a man without complaint,” as the Lord says, “and one that eschews every evil,” said, “Have I seen the sun when it shone or the moon walking brightly, and my heart has rejoiced in secret, and I have kissed my hand: what is my great iniquity and denial against the most High God?”
But what is the sun or what is the moon but elements of visible creation and material light: one of which is of greater brightness and the other of lesser light? For as it is now day time and now night time, so the Creator has constituted various kinds of luminaries, although even before they were made there had been days without the sun and nights without the moon. But these were fashioned to serve in making man, that he who is an animal endowed with reason might be sure of the distinction of the months, the recurrence of the year, and the variety of the seasons, since through the unequal length of the various periods, and the clear indications given by the changes in its risings, the sun closes the year and the moon renews the months.
For on the fourth day, as we read, God said: “Let there be lights in the firmament of the heaven, and let them shine upon the earth, and let them divide between day and night, and let them be for signs and for seasons, and for days and years, and let them be in the firmament of heaven that they may shine upon earth.”
Source: Sermons (New Advent)