Star position in the sky is defined by a pair of angles. These two angles - which refer to the celestial equator - are called declination (abbrev. d or Dec) and right ascension (a or RA). While d is given in degrees (from +90° at the celestial north pole to -90° at the south pole), a is usually given in hours (0 ... 24h). This is due to the observation technique of star transits, which cross the eyepiece of telescopes because of Earth's rotation. The observation techniques are topics of position astronomy and of astrogeodesy.