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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 eyepiece of telescopes because of the Earth's rotation. The observation techniques are topics of position astronomy and of astrogeodesy.
Star positions are changing in time, caused by
- precession and nutation - slow tilts of the Earth's axis with rates of 50 arcseconds and 2 arcseconds respectively, per year;
- aberration and parallax - effects of the Earth's orbit around the sun;
- 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).
- Star catalogue, FK4, FK6
- Equatorial coordinates, Ecliptic coordinates
- Vernal equinox, Yearbook
- annual aberration, improper motion
- Geodetic astronomy, transit instruments