Equatorial coordinates consist of a longitudinal Right Ascension (RA)
and a latitudinal Declination (Dec). The plane of zero declination is
the projection of the Earth's equator onto the celestial sphere, and the
zero of RA is marked by the intersection of the ecliptic (Earth
orbital) plane with the equatorial plane. This definition depends on
epoch because of precession; its practical implementation also depends
on the set of fundamental reference stars used to measure
coordinates. There are two fundamental frames of reference currently
in use: the FK4 and FK5 systems. The FK4 system is tied to the
equatorial coordinate system for Besselian epoch B1950.0, while the
more accurate FK5 system is tied to the equatorial system for Julian
epoch J2000.0. The International Astronomical Union (IAU) has
recommended that J2000.0 coordinates be used.
Note that since the FK4 and FK5 reference frames rotate relative to
one another, the transformation from B1950 to J2000 affects not only
the coordinates but the proper motions of stars. An object with zero
proper motion in B1950 will have a nonzero proper motion in J2000 and
vice versa. Since this program does not deal with proper motions,
coordinate conversions have been adopted which assume that the object
in question has zero proper motion in the J2000 system. Note further,
however, that the difference between J2000 and B2000 is typically less
than one arc second.
Defined conceptually by the Galactic plane and the Galactic center,
galactic longitude l and galactic latitude b are the IAU 1958 system,
formerly called lII and bII. The system is defined in terms of B1950
equatorial coordinates as RA of (l=0) = 192.25 degrees, inclination of
galactic equator to B1950 equator = 62.6 degrees, longitude of
ascending node 33 degrees.
Galactic l, b B1950 RA, Dec J2000 RA, Dec
----------------------------------------------------------------------
0 0 17 42 26.58 -28 55 00.43 17 45 37.20 -28 56 10.22
0 +90 12 49 00.00 +27 24 00.00 12 51 26.28 +27 07 41.70
33.0 0 18 49 00.00 00 00 00.00 18 51 33.73 +00 03 38.13
123.0 +27.4 12 00.00 00 +90 00 00.00 12 01 16.85 +89 43 17.74
The J2000 north celestial pole is at l=122.932, b=27.128.
With the advent of large scale structure studies, the supergalactic
coordinate system introduced by de Vaucouleurs is coming into more
widespread use. The supergalactic equator is conceptually defined by
the plane of the local (Virgo-Hydra-Centaurus) supercluster, and the
origin of supergalactic longitude is at the intersection of the
supergalactic and galactic planes. Supergalactic longitude and
latitude, SGL and SGB, are defined by
SGL, SGB Galactic l, b J2000 RA, Dec
--------------------------------------------------------
0 0 137.37 0 02 49 14.43 +59 31 42.05
0 +90 47.37 +6.32 18 55 00.98 +15 42 32.17
90.0 6.32 0 +90 12 51 26.28 +27 07 41.70
Ecliptic coordinates are defined by the earth's orbital plane. The
B1950 north ecliptic pole is at B1950 RA 18 00 00, Dec +66 33
15. Since ecliptic coordinates are weakly epoch dependent, you can use
a specific epoch to define the orbital plane other than the default
B1950.0. In PRECESS and COLDEN, to select Besselian epoch Bxxxx, use
the from/to option ECxxxx. Julian epochs are not provided. EC is
equivalent to EC1950.
B1950 coordinates are precessed to B1875.0 and compared with the
Delporte (1935) constellation definitions.
![[CIAO Logo]](../imgs/ciao_logo_navbar.gif){kind=logo}