Star position

The location is defined by a pair of angular coordinates relative to the celestial equator: right ascension (α) and declination (δ).

While δ is given in degrees (from +90° at the north celestial pole to −90° at the south), α is usually given in hour angles (0 to 24 h).

The following factors change star positions over time: The first and second effects are considered by so-called mean places of stars, contrary to their apparent places as seen from the moving Earth.

Absolute and very precise coordinates of 1000-3000 stars are collected in fundamental catalogues, starting with the FK (Berlin ~1890) up to the modern FK6.

Relative coordinates of numerous stars are collected in catalogues like the Bonner Durchmusterung (Germany 1859-1863, 342,198 rough positions[1]), the SAO catalogue (USA 1966, 250.000 astrometric stars) or the Hipparcos and Tycho catalogue (110.000 and 2 million stars by space astrometry).