In astronomy and astrometry, the spherical coordinates (usually only the two angular coordinates ) of stars on the imaginary celestial sphere are referred to as star locations (singular star location) .
A distinction is first made between relative locations and absolute locations . A relative location is given relative to a well-measured star ( fundamental star ), an absolute location directly in the equatorial coordinate system . The latter relates - analogous to the geographical latitude and longitude - to the celestial equator (projection of the earth's equator) and the spring equation (intersection of celestial equator and ecliptic ). The two coordinates are called declination (δ) and right ascension (α or RA).
The place at which a star appears on the celestial sphere ( observed place ) is influenced by the refraction of light ( refraction ), the movement of the earth (effects of aberration and parallax as well as precession and nutation ) and the position of the observer. Accordingly, a distinction is made between:
- Apparent star location : observed location, corrected for refraction and daily aberration, based on the center of the earth ( geocentric location )
- True star location : Apparent location, corrected for annual aberration and parallax, based on the center of the sun ( heliocentric location )
- Middle star location : True star location which, taking precession and nutation into account, is related to a specific point in time. A standard epoch is usually chosen as the point in time , e.g. B. J2000.0 . Between 1925 and 1990 the usual standard epoch was 1950.0 , prior to that 1920, 1900 and 1875.
The differentiation of the observer's location (heliocentric, geocentric or topocentric for a specific geographical location ) is particularly important for nearby objects such as those of the solar system.
Star catalogs contain information on the middle places.
Apparent star locations
As apparent star positions (engl. Apparent Places ) are referred to those star coordinates on observations on the apparent celestial sphere reflect or directly from angle measurements follow in the system of the equatorial coordinates. They are called "apparently" because they are influenced by the current position of the earth and its movement around the sun as well as by the earth's rotation and the slow displacement of the earth's axis . These effects are mainly some types of aberration and parallax, as well as precession and nutation of the earth's axis.
If the coordinates (mostly right ascension and declination) are freed from these influences, they are called mean star locations . These change more slowly than the apparent star locations, which is why they are used in particular for star catalogs and star maps . Today it is related to the point in time J2000.0 , while before 1990 the arithmetic epoch 1950.0 was common.
Middle star locations
The mean star locations are those star coordinates on the celestial sphere that do not refer to the time of observation but to a computational epoch.
As a result, the apparent star locations to be derived from angle measurements are freed from those influences that arise from the current position and movement of the earth around the sun, from the earth's rotation and the slow displacement of the earth's axis. Above all, these are some types of aberration and parallax, as well as precession and nutation of the earth's axis.
These mean star locations are usually related to the beginning or the middle of the year, or as a standard epoch for star catalogs and star maps to the point in time 2000.0. From around 1930–1990, 1950.0 was used as the arithmetic epoch and 1900.0 before that.
- Albert Schödlbauer : Geodetic Astronomy. Basics and concepts. De Gruyter, Berlin / New York 2000, ISBN 3-11-015148-0
- RM Green: Spherical Astronomy , Cambridge University Press, Cambridge 1985, ISBN 0-521-23988-5 and ISBN 0-521-31779-7
- The system presented here follows Hans-Heinrich Voigt: Abriss der Astronomie. BI-Wissenschaftsverlag, 5th, revised edition, Mahnnheim, Vienna, Zurich 1991, ISBN 3-411-15255-9 , pp. 11, 22.