In the symbolic language of the Church the Sun represents the Son of God, Jesus Christ, and the moon represents the Most Holy Theotokos.

Revelation 12:1 And a great portent appeared in heaven, a woman clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars; 2 she was with child and she cried out in her pangs of birth, in anguish for delivery.

The imagery of the moon is that of reflection of light. The moon reflects the light from the sun, just as we are called to reflect the glory of God.

Wisdom of Solomon 7:26 (this chapter is the background for Revelation 12) For she is a reflection of eternal light, a spotless mirror of the working of God, and an image of his goodness.