The Book of Zohar, Chapter “Beresheet (Genesis),” Item 113, The Two Great Lights: The moon was not at peace with the sun for she was ashamed of her. … The Creator told her, “Go and diminish yourself.”
We are talking about the adaptation between the created being Malchut and the Creator, the first nine Sefirot by which the Creator is depicted to the created being. According to the degree that Malchut adapts to the first nine Sefirot, it reaches adhesion with them, pursuant to the principle of “By Your actions, we know You.”
The adhesion is realized by equivalence of form, balance, or homeostasis between two objects as we call it in our world.
Although one of the two great luminaries Malchut has no light of its own, it is still great and shiny, and can be as important as the sun, the Creator, because although by itself it dwells in darkness, it can go on to shine.
The desire to receive is called the Earth. Its level of bestowal is the moon. The force it gets in order to bestow, to correct itself, and shine at night, comes from the sun, from Zeir Anpin in which there is Light from upper Bina.
So, there are two lights, the small light and the great light. The purpose of the development of the small light is to reach the same size, the same light, as the great light so that there will be two great lights. In this way, the created being will reach the same level, perception, and consciousness, and everything that the Creator wants for him. This is the adhesion of the created being with the Creator.
From the 2nd part of the Daily Kabbalah Lesson 1/8/12, The Zohar