- Star position
The position of stars on the sky are defined by a couple of
angle s, similar to thegeographic latitude andlongitude . These two angles - which refer to thecelestial equator - are called "Declination " (abbrev. "δ" or "Dec") and "Right ascension " ("α" or "RA").
.Ideally the two-dimensional
coordinate system "α, δ" refers to aninertial frame of reference ; the 3rd coordinate is the stardistance which normally is used as anattribute of the individual star.Star positions are changing in time, caused by
#precession andnutation - slow tilts of the Earth's axix with rates of 50" resp. 2" per year
#aberration andparallax - 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 specialepoch , e.g. 1950.0 or2000.0 . The 3rd effect has to be handled individually.The star positions "α, δ" are compiled in several
star catalogue s of different volume and accuracy. "Absolute" and very precise coordinates of 1000-3000 stars are collected inFundamental catalogue s, starting with the FK (Berlin ~1890) up to the modernFK6 .
"Relative" coordinates of numerous stars are collected in Catalogues like theBonner Durchmusterung (Germany 1852-1862, 200.000 rough positions), the SAO catalogue (USA 1966, 250.000 astrometric stars) or theHipparcos and Tycho catalogue (110.000 and 2 million stars by space astrometry).See also
*
Star catalogue ,FK4 ,FK6
*Equatorial coordinates ,Ecliptic coordinates
*Vernal equinox ,Yearbook
*annual aberration ,apparent motion
*Geodetic astronomy ,transit instrument sExternal links
* [http://www.telescope.org/information/astronomy/positionofstars.php How Astronomers describe the position of stars]
* [http://books.google.at/books?id=0Srje-dQBicC&pg=RA3-PA261&lpg=RA3-PA261&dq=%22star+position%22+astrometry&source=web&ots=EQrDuai_Ho&sig=Tkcz72tfnYORNRKfWv_9gcfuc9s&hl=de J.Kovalevsky, Fundamentals of Astrometry]
* [http://www.ari.uni-heidelberg.de/publikationen/apf/ Apparent Places of Fundamental Stars]
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