Earth’s axis wobbles like a spinning top, and so the point where the sun rises at the spring equinox creeps backward through the zodiac — about one degree every 72 years, a full circuit in roughly 25,800 years. That vast cycle is the ‘Great Year’, and the shift of the equinox from one constellation to the next is what astrologers call an ‘Age’. Slide the millennia and watch the pole circle.
Axial precession: gravitational torque (mostly Sun and Moon) on Earth’s equatorial bulge makes the spin axis trace a cone, so the equinox point regresses along the ecliptic at ~50.3″/year ≈ 1°/71.6 yr, completing a circle (the platonic ‘Great Year’) in ~25,772 years. Each ~2,150-year span the equinox spends in one zodiac constellation is an astrological ‘Age’. Hipparchus discovered it c. 127 BCE. A fail-loud self-check throws unless 360×~72 yr gives a Great Year of 25,000–26,500 years. ◆ real astronomy, node-verified.
LIT: the precession and its ~25,772-year period are exact, observed astronomy. AMBER: the meaning of the ‘Ages’ (Aquarius, etc.) is astrological belief; even the boundaries depend on where you draw the (unequal) constellations.