PrecessionThe 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.
Right ascensionThe celestial “longitude”, counted eastwards along the celestial equator from the vernal equinox point, in hours from 0h to 24h (1h = 15°). With declination it fixes a star’s position on the celestial sphere. Abbreviated RA.
DeclinationThe celestial “latitude”: an object’s angle north (+) or south (−) of the celestial equator, from +90° to −90°. Vega is at +38.8°, Sirius at −16.7°. From a given latitude, declination tells you whether an object is always up, never up, or rises and sets.