What phenomenon caused the shift in constellation boundaries relative to the equinoxes over two millennia?
Precession of the equinoxes
The misalignment between the ancient astrological dates and the actual current positions of the Sun among the stars is directly attributed to the precession of the equinoxes. This astronomical effect involves a slow, cyclic wobble in the orientation of Earth's axis of rotation, much like a spinning top slowing down. Because the initial boundaries of the constellations were established around two thousand years ago, setting the sign boundaries to align perfectly with the equinoxes and solstices of that era, this continuous, gradual axial wobble has subsequently caused the alignment between the tropical year markers and the fixed stellar patterns to shift by approximately the width of a full constellation over that long period.
