star Stellar positionsastronomy

Accurate observations of stellar positions are essential to many problems of astronomy. Positions of the brighter stars can be measured very accurately in the equatorial system (the coordinates of which are called right ascension [α, or RA] and declination [δ, or DEC] and are given for some epoch—for example, 1950.0 or, currently, 2000.0). Fainter stars are measured by using photographic plates or electronic imaging devices (e.g., a charge-coupled device, or CCD) with respect to the brighter stars, and finally the entire group is referred to the positions of known external galaxies (see galaxy). These distant galaxies are far enough away to define an essentially fixed, or immovable, system, whereas positions of both the bright and faint stars are affected over relatively short periods of time by galactic rotation and by their own motions through the Galaxy.