# Epoch (J2000)

Also called: J2000, precession.

The date that coordinates refer to — usually J2000, the start of the year 2000 — because the sky's grid slowly shifts.

The Earth's axis wobbles like a spinning top over 26,000 years (precession), so the grid of right ascension and declination slides slowly against the stars. Catalogs give coordinates for a standard epoch, J2000; for the next few decades the difference is a small fraction of a degree.

**Related:** [Right ascension](https://astropilot.space/glossary/right-ascension.md), [Declination](https://astropilot.space/glossary/declination.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/epoch