Everything about Star Position totally explained
The
position of stars on the sky are defined by a couple of
angles, similar to the
geographic latitude and
longitude. These two angles - which refer to the
celestial equator - are called
Declination (abbrev.
δ or
Dec) and
Right ascension (
α or
RA).
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.
Ideally the two-dimensional
coordinate system α, δ refers to an
inertial frame of reference; the 3rd coordinate is the star
distance which normally is used as an
attribute of the individual star.
Star positions are changing in time, caused by
- precession and nutation - slow tilts of the Earth's axix with rates of 50" resp. 2" 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, for example 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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