26000year

The term 26000year refers to the precession of the equinoxes, the slow wobble of Earth's rotation axis. This axial procession causes the celestial coordinates of stars to shift gradually relative to the seasons, so the vernal equinox slowly moves westward along the ecliptic. The cycle completes in about 25,700 to 26,000 years, which is why popular references often round it to roughly 26,000 years.

The effect arises from gravitational torques exerted by the Moon and Sun on Earth's equatorial bulge. Over many millennia this torque changes the orientation of Earth's axis in space. The rate is about 50.3 arcseconds per year, yielding a full 360-degree revolution in roughly 25,770 years.

Hipparchus of Nicaea is traditionally credited with discovering the phenomenon in the 2nd century BCE, noting a gradual shift in star positions since earlier Greek observations. In modern astronomy, precession is described relative to a fixed celestial reference frame. It is distinct from shorter-term motions such as nutation, which superimpose small oscillations on top of the general drift.

Historically, the cycle has been called the Great Year or Platonic Year in ancient and classical writings. In astrology it is associated with astrological ages (for example, the transition from Pisces to Aquarius), though the exact boundaries and meanings are debated and not part of mainstream astronomy.

Today precession is a standard consideration in celestial coordinate systems, long-term celestial mapping, and navigational astronomy, as it gradually alters the positions of stars and the timing of equinoxes over centuries.