right ascension and declination

To find a friend's house in a city you need an address — a street and a number. To find a star, astronomers need a sky address that everyone agrees on and that does not depend on where you are standing or what time it is. Right ascension and declination are that address system: the celestial equivalent of longitude and latitude, fixed to the sky itself.

Declination (Dec) is the celestial latitude: how far a star lies north or south of the celestial equator, measured in degrees from +90 degrees at the north celestial pole, through 0 at the equator, to -90 degrees at the south pole. Right ascension (RA) is the celestial longitude: how far around the sky a star lies, measured eastward from a fixed reference point (the vernal equinox, where the Sun crosses the equator in spring). RA is traditionally given not in degrees but in hours, minutes, and seconds — 24 hours make a full circle — which neatly matches the sky's daily rotation, since one hour of RA passes overhead each hour. Together, an RA and a Dec pin any object to one spot on the celestial sphere.

These coordinates are the universal language of where things are in the sky. A star catalogue, a telescope's pointing computer, and an observing plan all speak in RA and Dec, so an object found in one observatory can be located instantly from another. One honest caveat: because the Earth's axis slowly wobbles (precession), the reference point drifts over centuries, so coordinates are always tagged with an epoch — a date, such as J2000.0 — for which they are exact.