Star position

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The spherical star coordinate system

Star position in the sky is defined by a pair of angles relative to the celestial equator: declination (δ) and right ascension (α).

While δ is given in degrees (from +90° at the celestial north pole to −90° at the south pole), α is usually given in hours (0 ... 24h). This is due to the observation technique of star transits, which cross the field of view of telescope eyepieces because of Earth's rotation. The observation techniques are topics of positional astronomy and of astrogeodesy.

Ideally the two-dimensional coordinate system α, δ refers to an inertial frame of reference; the 3rd coordinate is the star distance, which is normally used as an attribute of the individual star.

Star positions are changing in time, caused by

  1. precession and nutation – slow tilts of Earth's axis with rates of 50 arcseconds and 2 arcseconds respectively, per year;
  2. aberration and parallax – effects of Earth's orbit around the Sun;
  3. proper motion of the individual stars.

The effects 1 and 2 are considered by so-called mean places of stars, contrary to their apparent places as seen from the moving Earth. Usually the mean places refer to a special epoch, e.g. 1950.0 or 2000.0. The 3rd effect has to be handled individually.

The star positions α, δ are compiled in several star catalogues of different volume and accuracy. 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 1852-1862, 200.000 rough positions), the SAO catalogue (USA 1966, 250.000 astrometric stars) or the Hipparcos and Tycho catalogue (110.000 and 2 million stars by space astrometry).

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