The Earth’s axis slowly sweeps out a cone, like a spinning top, in about 26,000 years. The celestial poles therefore wander among the stars: in about 12,000 years Vega will be near the north pole. Star coordinates slowly change, which is why an epoch such as J2000 is given.
Related terms
- Epoch (J2000)
The date to which celestial coordinates refer, since they slowly change with precession. J2000 means 1 January 2000 at 12:00; it is the reference for today’s charts and catalogues. GoTo mounts convert these to coordinates “of date”.
- Celestial poles
The two points in the sky around which all stars seem to turn: the extension of the Earth’s axis. The north celestial pole lies very close to Polaris; the south pole has no bright star. The pole’s height above the horizon equals your latitude.
- Equinox
The moment the Sun crosses the celestial equator, around 20 March and 22 September; day and night then last about 12 hours everywhere. The point the Sun crosses in March, the vernal equinox point, is the zero of right ascension. On that day the Sun rises due east.
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