RADec

RADec, short for Right Ascension and Declination, is the most common celestial coordinate system used to specify the positions of objects on the sky. It is analogous to longitude and latitude on Earth and is anchored to the celestial equator and the vernal equinox, providing a two-dimensional map of the celestial sphere.

Right Ascension is the angular distance measured eastward along the celestial equator from the vernal equinox. It is typically expressed in hours, minutes, and seconds, with 24 hours constituting a full circle. Since 1 hour equals 15 degrees, RA acts as a time-like coordinate tied to Earth's rotation and sidereal time rather than a fixed angular measure alone.

Declination is the angular distance north or south of the celestial equator, expressed in degrees, arcminutes, and arcseconds. Its range is from +90 degrees (north celestial pole) to −90 degrees (south celestial pole). Together, RA and Dec locate a point on the celestial sphere for a given epoch.

Coordinates are defined for a specific reference epoch, with J2000.0 being the most widely used standard in modern catalogs. Precession, nutation, and proper motion cause small changes over time, so the epoch must be specified and, if necessary, transformations performed to other frames such as the International Celestial Reference System (ICRS) or to different epochs.

RADec is integral to telescope pointing, object catalogs, sky surveys, and cross-matching astronomical data. Objects maintain fixed RA and Dec coordinates for a given epoch, though their observed positions can change slightly over time due to motion and the precession of the Earth's axis. Conversions to other coordinate systems, like galactic or ecliptic coordinates, are routine in astronomical software.