[The Moon Says:]

He made the moon for festivals,

the sun knows its setting.

(Tehillim 104:19)

G-d created the moon to be the basis of our calendar, the timeline of the Festivals.  The moon does not have its own light, it shines in conjunction with the sun.  But "the sun knows it setting."  Its rising and setting are fixed because it shines by its own light.  The waning and waxing of the moonlight symbolizes human life.  Until his middle years, a person continues to increase, thereafter he continues to decrease, until he reaches the appointed place of all living things.
[MeAm Lo'ez, Perek Shira]