SUBROUTINE iau_ICRS2G ( DR, DD, DL, DB )
*+
* - - - - - - - - - - -
* i a u _ I C R S 2 G
* - - - - - - - - - - -
*
* Transformation from ICRS to Galactic Coordinates.
*
* This routine is part of the International Astronomical Union's
* SOFA (Standards of Fundamental Astronomy) software collection.
*
* Status: support routine.
*
* Given:
* DR d ICRS right ascension (radians)
* DD d ICRS declination (radians)
*
* Returned:
* DL d galactic longitude (radians)
* DB d galactic latitude (radians)
*
* Notes:
*
* 1) The IAU 1958 system of Galactic coordinates was defined with
* respect to the now obsolete reference system FK4 B1950.0. When
* interpreting the system in a modern context, several factors have
* to be taken into account:
*
* . The inclusion in FK4 positions of the E-terms of aberration.
*
* . The distortion of the FK4 proper motion system by differential
* Galactic rotation.
*
* . The use of the B1950.0 equinox rather than the now-standard
* J2000.0.
*
* . The frame bias between ICRS and the J2000.0 mean place system.
*
* The Hipparcos Catalogue (Perryman & ESA 1997) provides a rotation
* matrix that transforms directly between ICRS and Galactic
* coordinates with the above factors taken into account. The
* matrix is derived from three angles, namely the ICRS coordinates
* of the Galactic pole and the longitude of the ascending node of
* the galactic equator on the ICRS equator. They are given in
* degrees to five decimal places and for canonical purposes are
* regarded as exact. In the Hipparcos Catalogue the matrix elements
* are given to 10 decimal places (about 20 microarcsec). In the
* present SOFA routine the matrix elements have been recomputed from
* the canonical three angles and are given to 30 decimal places.
*
* 2) The inverse transformation is performed by the routine iau_G2ICRS.
*
* Called:
* iau_ANP normalize angle into range 0 to 2pi
* iau_ANPM normalize angle into range +/- pi
* iau_S2C spherical coordinates to unit vector
* iau_RXP product of r-matrix and p-vector
* iau_C2S p-vector to spherical
*
* Reference:
* Perryman M.A.C. & ESA, 1997, ESA SP-1200, The Hipparcos and Tycho
* catalogues. Astrometric and photometric star catalogues
* derived from the ESA Hipparcos Space Astrometry Mission. ESA
* Publications Division, Noordwijk, Netherlands.
*
* This revision: 2021 January 25
*
* SOFA release 2021-05-12
*
* Copyright (C) 2021 IAU SOFA Board. See notes at end.
*
*-----------------------------------------------------------------------
IMPLICIT NONE
DOUBLE PRECISION DR, DD, DL, DB
DOUBLE PRECISION iau_ANP, iau_ANPM
DOUBLE PRECISION V1(3), V2(3)
*
* L2,B2 system of galactic coordinates in the form presented in the
* Hipparcos Catalogue. In degrees:
*
* P = 192.85948 right ascension of the Galactic north pole in ICRS
* Q = 27.12825 declination of the Galactic north pole in ICRS
* R = 32.93192 longitude of the ascending node of the Galactic
* plane on the ICRS equator
* R = 32.93192 Galactic longitude of the ascending node of
* the Galactic equator on the ICRS equator
*
* ICRS to galactic rotation matrix, obtained by computing
* R_3(-R) R_1(pi/2-Q) R_3(pi/2+P) to the full precision shown:
*
DOUBLE PRECISION R(3,3)
DATA R(1,1), R(1,2), R(1,3),
: R(2,1), R(2,2), R(2,3),
: R(3,1), R(3,2), R(3,3) /
: -0.054875560416215368492398900454D0,
: -0.873437090234885048760383168409D0,
: -0.483835015548713226831774175116D0,
: +0.494109427875583673525222371358D0,
: -0.444829629960011178146614061616D0,
: +0.746982244497218890527388004556D0,
: -0.867666149019004701181616534570D0,
: -0.198076373431201528180486091412D0,
: +0.455983776175066922272100478348D0 /
* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
* Spherical to Cartesian.
CALL iau_S2C ( DR, DD, V1 )
* ICRS to Galactic.
CALL iau_RXP ( R, V1, V2 )
* Cartesian to spherical.
CALL iau_C2S ( V2, DL, DB )
* Express in conventional ranges.
DL = iau_ANP ( DL )
DB = iau_ANPM ( DB )
* Finished.
*+----------------------------------------------------------------------
*
* Copyright (C) 2021
* Standards Of Fundamental Astronomy Board
* of the International Astronomical Union.
*
* =====================
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* =====================
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*-----------------------------------------------------------------------
END