Read St. Louis de Montfort's work "True Devotion To Mary". He refers to the BVM as the moon for her gentle light that's actually from the source of Life and Light, her son Jesus Christ (the sun's rays give light to the moon). When sinners are too fearful to go to the all powerful Light of Christ, they are comforted by the gentle light of Mary as she directs sinners to her Son - the Savior of God's children. When the sun's rays are too powerful by day in which to navigate, not out of fault of the sun but rather due to the fallen nature of man, the navigator will use the gentle light of the moon to navigate through darkness to the Destination (Christ's salvation). There are other OT verses prophesizing the mother of Our Savior by referring to her as the "moon". They were also prefigurements for the later birth and life of Mary, and her role as Queen of the Church and Heaven.