Synopsis of HYP, Chapter 1, vs 1-3
Many people try to accomplish Royal Union without first having accomplished Sun-Moon Union. This is not possible. The author tells us that this error is made by people because of the darkness caused by the multiplicity of religions and belief systems. So after paying homage to the First Master who taught Sun-Moon Union, he reveals these teachings out of compassion. The sole purpose of Sun-Moon Union (Hatha Yoga) is for the attainment of Royal Union (Raja Yoga).
Read the Mystical Tidbit from which this synopsis is derived, here.
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What are these two forms of union? This question is not answered in these verses, but here is the short version:
When the sun, the warming energy in the body, and the moon, the cooling energy in the body, unite at the base of the spine, the evolutionary force is released. The tendency of this evolutionary force is to rise upward through a central pathway. When this is allowed to occur and the evolutionary force reaches a particular place in the head, this is called Royal Union.
Royal Union can be understood as the union, merging, or marriage, of purusha (you with a point of view) and prakriti (what is viewed, or perceived—the divine other-than-you); of the knower and the known; of you and God. Royal Union is the Highest union, the Highest Yoga.
Royal Union is a process of moving from the achievement of this special state to a point in which one is able to live in this state.