It's interesting to me that in Genesis 1, the moon is called a light. Skeptics have tried to use this to discredit the Bible by saying ridiculous things like, "Ha! See, the writer didn't even know that the moon is merely a reflector of the sun, not a light in and of itself!" Obviously, God was referring to the function of the moon as the means of giving off light at night; He was not going into scientific detail about how the light is produced. God was referring to the fact that He made the moon to be a light in the darkness. The moon was made to reflect the light of the sun.
"Ye are the light of the world..." In the same way that the moon has absolutely no power to give off light apart from the sun, we cannot give off the light of the Gospel apart from the Son. 2 Corinthians 4:6 reads, "But God, who commanded the light to shine out of darkness, hath shined in our hearts..." We are not the source of the light; we are merely reflectors of the light that has shined in our hearts. Isn't it interesting that in both instances, something that is not a light in and of itself is called a light by God? The moon was made to give off the sun's light to penetrate physical darkness; we as Christians were made to give off the Son's light to penetrate spiritual darkness.