era.g2icrs.md

eraG2icrs

[dr, dd] = ERFA.g2icrs( dl, db)

Transformation from Galactic Coordinates to ICRS.

Given:

   dl     double      galactic longitude (radians)
   db     double      galactic latitude (radians)

Returned:

   dr     double      ICRS right ascension (radians)
   dd     double      ICRS declination (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 eraIcrs2g.

Called:

   eraAnp       normalize angle into range 0 to 2pi
   eraAnpm      normalize angle into range +/- pi
   eraS2c       spherical coordinates to unit vector
   eraTrxp      product of transpose 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: 2021 January 25

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.