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# icrs2g

erfa.icrs2g(dr, dd)

  Transformation from ICRS to Galactic coordinates.

  Parameters:
    dr double array
    dd double array
  Returns:
    dl double array
    db double array

  Notes

  Wraps ERFA function `eraIcrs2g`. The ERFA documentation is:

      - - - - - - - - - -
       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.