Every location on Earth has a latitude and longitude coordinate.
Latitude measures how far north or south of the Earth's equator a place is. Longitude measures how far west or east a place is from a north-south line called the prime meridian, which
runs through .Greenwich, England.
Similarly, every object in the sky can be mapped with two coordinates: declination and right ascension. Declination (dec.) measures the object's position north (+) or south (-) of the celestial equator, a circle that divides the sky into northern and southern hemispheres. Declination is measured in degrees, minutes, and seconds of arc. There are 60 arcseconds in an arc- , minute, and 60 arcminutes in a degree. An object on the celestial equator has a declination of 0 degrees. The distance from then celestial equator to either of the two celestial poles, is 90 degrees.
Right ascension (RA) measures how far east an object lies from the sky's prime meridian. The prime meridian is an imaginary line-running due north and south through the vernal equinox—the point where the Sun crosses the celestial equator on 20 March each year. Right ascension is given in hours, minutes, and seconds oftf'me. It is measured eastward from the prime meridian, which has an RA of 0 hours, until we reach 23 hours, 59 minutes.
While all celestial objects have right ascension and declination coordinates, these numbers slowly
change because of a motion of the Earth called precession. As a result, catalogs and star charts specify which "epoch" they are designed for—most currently in use give coordinates accurate for the year 2000.