Step-by-step explanation:
Hipparchus had observed and discovered the precession motion of Earth. Due to the precession the axis of the Earth will gradual shift in a cycle of 25772 years. That means the axis will point towards different stars in the celestial sphere and our pole stars will change.
Because of this change in the axis the apparent position of Sun with respect to the background constellation (sun signs) regresses. Because of which the dates associated with the sun signs will be wrong.