Declination and Right Ascension
Declination (DEC) is the celestial sphere's equivalent of latitude and it is expressed in degrees, as is latitude. For DEC, + and - refer to north and south, respectively. The celestial equator is 0° DEC, and the poles are +90° and -90°.
Right ascension (RA) is the celestial equivalent of longitude. RA can be expressed in degrees, but it is more common to specify it in hours, minutes, and seconds of time: the sky appears to turn 360° in 24 hours, or 15° in one hour. So an hour of RA equals 15° of sky rotation.
Another important feature intersecting the celestial sphere is the ecliptic plane. This is the plane in which the Earth orbits the sun, 23.4° from the celestial equator. The great circle marking the intersection of the ecliptic plane on the celestial sphere is where the sun and planets appear to travel, and it's where the Sun and Moon converge during their eclipses (hence the name).
The zero point for RA is one of the points where the ecliptic circle intersects the celestial equator circle. It's defined to be the point where the sun crosses into the northern hemisphere beginning spring: the vernal equinox, also known as the first point of Aries.