A person's sidereal birthday is marked by a person's "Nakshatra" which points to a division in the observational sky. Ancient Skies and Astronomy Now, dives deeper into concepts behind tropical and sidereal astronomy.
Moon serves as a metaphor to an existence that is derivative or a reflection of a primary entity. Moonshine is radiation from the Sun. Mind is an abstraction which exists while life force moves within the body. Mind is self similar to Moon in Yoga tradition. The moon arriving back in its original phase to its original position in the sky, in a mystical way, is a reminder of the birth of mind while the Sun, a metaphor for life force, returning to its original position is a reminder of the birth of body.