The day of the full moon "Amazingly, the day of the full moon is the one day of the month when the moon rises at precisely sunset and sets at precisely sunrise! This indicates that the day of the full moon is a day when both luminaries exist in harmony; each one exits in order for the other to have a grand solo entrance! They give honor to each other on the day of the full moon.
"Both Passover and Sukkos begin on the Fifteenth. As we sit at the Passover Seder, a full moon watches over the house. As we sit in the sukkah on the first night of Sukkos, a full moon is visible through the sechach branches covering the sukkah.
"Why all this glory on the Fifteenth? The sun and the moon are giving honor to each other! One exits while the other enters, making way for each other. Is this not the secret of the Jewish people's joy on Passover and Sukkos? All our joy is based on the honor we give to each other as one united family. Did we not stand at Mount Sinai "k'ish echad b'lev echad … like one man with one heart?" (Rashi on Exodus 19:2) We are in exile because of baseless hatred; we will be saved by baseless love. The Jewish People will be redeemed in the merit of our unity, and we learn this from the sun and the moon on the Fifteenth day of the month, the day the moon is full."
This seems to be an important insight into the power of the Fifteenth Day. On this day the sun and the moon extend each other honor, but particularly the moon, because the sun does not change, but the moon does change. We learn from our sages that the moon was diminished at the beginning of history when it said to G-d that there cannot be "two kings utilizing the same crown" (Chulin 60b). G-d then diminished the moon. Presumably, before that time its appearance was constant, like the sun's. But at that time it began its cycle of waxing and waning, in which it reappears on Rosh Chodesh, grows full at the Fifteenth and then diminishes each succeeding night until it disappears at the end of the month.
That waxing and waning is said to resemble the rise and fall of The Jewish People's fortunes during exile. Presumably, when exile ends, the moon will no longer vary in appearance, and this is borne out by what we say at the Kiddush Levana prayer, "May it be Your will, G-d … to fill the flaw in the moon that there be no diminution in it. May the light of the moon be like the light of the sun … as it was before it was diminished … and may there be fulfilled upon us the verse that is written: 'They shall seek G-d … and David, their King, Amen!'"
The moon is, by definition, secondary to the sun. The sun is the source of light, heat and energy, the center around which the solar system revolves. "The sun … is like a groom coming forth from his bridal chamber, rejoicing like a warrior to run the course. The end of the heavens is its source, and its circuit is to their other end; nothing is hidden from its heat." This is followed by, "The Torah of G-d is trustworthy," implying a connection between the constancy of the sun and the perfection of the Torah (Psalm 19). The moon, on the other hand, is the receiver, as opposed to the giver. Its glow comes from the sun.
The relationship between these two heavenly bodies is analogous to the relationship between G-d and the universe He created. All life radiates from Him; we are receivers. He is like the sun and we are like the moon. He is constant and unwavering; we are unstable, in a constant internal and external battle for life. Sometimes we are strong and bright, sometimes weak and dark.
When the moon says that two kings cannot utilize the same crown, G-d tells the moon "diminish yourself!" Something similar happened when Eve rebelled against G-d in the Garden of Eden. She was implicitly criticizing the way G-d had set up the universe. This is like the moon telling G-d that He had set up the universe deficiently. | 
