The crescent symbolizes a couple of things. 1) It represents reflected light. 2) It's a symbol of Shekinah in transcendence, the Moon of heaven as opposed to the Moon of earth. From the
Pictorial Key:
"In a manner, she is also the Supernal Mother herself—that is to say, she is the bright reflection. It is in this sense of reflection that her truest and highest name in symbolism is Shekinah—the cohabiting glory. According to Kabalism, there is a Shekinah both above and below. In the superior world it is called Binah, the Supernal Understanding which reflects to the emanations that are beneath. In the lower world it is Malkuth—that world being, for this purpose, understood as a blessed Kingdom—that with which it is made blessed being the Indwelling Glory."
The symbolism in Christian iconography comes from Revelation 12:1, "And there appeared a great wonder in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars." If you read the full passage you'll see it refers to Israel—in a general sense—giving birth to the Christ child; and to Mary in a more specific sense (at least that's my interpretation). This passage of scripture has been interpreted mystically in a variety of ways. Catholics see the woman as Mary, Queen of Heaven. This isn't too far from Waite's "Queen of the borrowed light" or Shekinah above.
