The Moon is Binary in Nature

Having heard the poem, Monkey went up to him and said, "Master, you only know about the moon's beauty, and you're homesick too. You don't know what the moon's really about. It's like the carpenter's line and compasses??it keeps the heavenly bodies in order. On the thirtieth of every month the metal element of its male soul has all gone, and the water element of its female soul fills the whole disk. That is why it goes black and has no light. That's what is called the end of the old moon. This is the time, between the last day of the old moon and the first of the new, when it mates with the sun. The light makes it conceive. By the third day the first male light is seen, and on the eight day the second male light. When the moon's male and female souls each have half of it, the moon is divided as if by a string. That is why it is called the first quarter. On the fifteenth night, tonight, all three male lights are complete, and the moon is round. This is called the full moon. On the sixteenth the first female principle is born, followed on the twenty?second by the second. At this stage the two souls are matched again and the moon is again divided as if by a string. This is what is called the third quarter. By the thirtieth the three female principles are complete, and it is the last day of the old moon. This is what is meant by 'prenatal absorption and refinement'. If we are all able gently to raise the 'double eight' and achieve it in nine by nine days, it will be easy to see the Buddha and easy to go home again too.

Old Chinese way of thinking about the moon, with the bright side as yang and the dark side yin.