LearnMechanics & orbitsSome maths
Precession of the equinoxes
The slow conical wobble of Earth's rotation axis, one turn every 25,772 years, driven by the Sun and Moon pulling on our equatorial bulge.
Why it matters
It is why catalogues carry an epoch. Coordinates given for J2000 are already visibly wrong for today's telescope pointing, and why Polaris has not always been the pole star.
In practice
Every plate solve on this site returns J2000 coordinates. Your mount converts to apparent-of-date before it slews, and that conversion is precession.
Rests on
2 concepts in the full chain back to first principles.
Leads to
Nothing here builds on it yet.