The Mosaic narrative gives the true purpose of the Sun and Moon.

Let us forsake then those doctrines of the Manichaeans, because they are the only witnesses concerning them, and let us hear those of Moses, to which all nations under Heaven bear witness, and in old time the Hebrews who reckoned according to the Moon, and after them all nations who are called Barbarians, and also the Greeks, who use the reckoning of the Sun, though they do not desert the reckoning of the Moon. And, therefore, even if we prolong our discourse, let us declare what is numbered by Sun-reckoning and what by Moon-reckoning. Days are numbered by Sun-reckoning. [P. 22, l.22.] For the dawning and darkness are indicated by the Sun. Behold the division of the day. But by the Moon the months are indicated. For the beginning of the months and end of the months are indicated by the Moon. [The Sun marks the days not the months.] For it is by the rising of the Sun and the setting of the Sun that the days are divided. But in the matter of months it makes no division, because its succession goes on uniformly, and does not declare any division when thirty days are ended, that it may be known by that division that the month has ended, or begun. [The Moon marks the months not the days.] But the Moon, when it is full and wanes, makes a division for the months, but makes no division for the days. For how often does it happen that the Moon rises at the third or fourth hour, and sets [P. 23, l. 2.] at the seventh or ninth hour ; while for two whole days she is not seen at all. God, in His wisdom who, indeed, ordered the months for the purpose of reckoning and the days for the purpose of numbering, made the Sun to number the days, as also the Moon to number the months, and as the day is completed in its course, so the Moon also is completed in its months, and from its beginning to its end the Moon produces thirty days. But if the day consists of twelve hours, and the Sun moves through a course of twelve hours, it is clear that the Sun is the fount of days. And, again, [lx] if the month consists of thirty days and the Moon completes thirty days in waning and waxing, it is clear that the Moon is the mother and parent of the months.