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icrs2g.c
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icrs2g.c
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#include "erfa.h"
void eraIcrs2g ( double dr, double dd, double *dl, double *db )
/*
** - - - - - - - - - -
** e r a I c r s 2 g
** - - - - - - - - - -
**
** Transformation from ICRS to Galactic coordinates.
**
** Given:
** dr double ICRS right ascension (radians)
** dd double ICRS declination (radians)
**
** Returned:
** dl double Galactic longitude (radians)
** db double 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 ERFA function 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 function eraG2icrs.
**
** Called:
** eraAnp normalize angle into range 0 to 2pi
** eraAnpm normalize angle into range +/- pi
** eraS2c spherical coordinates to unit vector
** eraRxp product of r-matrix and p-vector
** eraC2s 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: 2023 April 16
**
** Copyright (C) 2013-2023, NumFOCUS Foundation.
** Derived, with permission, from the SOFA library. See notes at end of file.
*/
{
double 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 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 r[3][3] = { { -0.054875560416215368492398900454,
-0.873437090234885048760383168409,
-0.483835015548713226831774175116 },
{ +0.494109427875583673525222371358,
-0.444829629960011178146614061616,
+0.746982244497218890527388004556 },
{ -0.867666149019004701181616534570,
-0.198076373431201528180486091412,
+0.455983776175066922272100478348 } };
/* Spherical to Cartesian. */
eraS2c(dr, dd, v1);
/* ICRS to Galactic. */
eraRxp(r, v1, v2);
/* Cartesian to spherical. */
eraC2s(v2, dl, db);
/* Express in conventional ranges. */
*dl = eraAnp(*dl);
*db = eraAnpm(*db);
/* Finished. */
}
/*----------------------------------------------------------------------
**
**
** Copyright (C) 2013-2023, NumFOCUS Foundation.
** All rights reserved.
**
** This library is derived, with permission, from the International
** Astronomical Union's "Standards of Fundamental Astronomy" library,
** available from http://www.iausofa.org.
**
** The ERFA version is intended to retain identical functionality to
** the SOFA library, but made distinct through different function and
** file names, as set out in the SOFA license conditions. The SOFA
** original has a role as a reference standard for the IAU and IERS,
** and consequently redistribution is permitted only in its unaltered
** state. The ERFA version is not subject to this restriction and
** therefore can be included in distributions which do not support the
** concept of "read only" software.
**
** Although the intent is to replicate the SOFA API (other than
** replacement of prefix names) and results (with the exception of
** bugs; any that are discovered will be fixed), SOFA is not
** responsible for any errors found in this version of the library.
**
** If you wish to acknowledge the SOFA heritage, please acknowledge
** that you are using a library derived from SOFA, rather than SOFA
** itself.
**
**
** TERMS AND CONDITIONS
**
** Redistribution and use in source and binary forms, with or without
** modification, are permitted provided that the following conditions
** are met:
**
** 1 Redistributions of source code must retain the above copyright
** notice, this list of conditions and the following disclaimer.
**
** 2 Redistributions in binary form must reproduce the above copyright
** notice, this list of conditions and the following disclaimer in
** the documentation and/or other materials provided with the
** distribution.
**
** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
** the International Astronomical Union nor the names of its
** contributors may be used to endorse or promote products derived
** from this software without specific prior written permission.
**
** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
** POSSIBILITY OF SUCH DAMAGE.
**
*/