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# atic13

erfa.atic13(ri, di, date1, date2)

  Transform star RA,Dec from geocentric CIRS to ICRS astrometric.

  Parameters
    ri double array
    di double array
    date1 double array
    date2 double array
  Returns
    rc double array
    dc double array
    eo double array

  Notes

  Wraps ERFA function `eraAtic13`. The ERFA documentation is:

      - - - - - - - - - -
       e r a A t i c 1 3
      - - - - - - - - - -

      Transform star RA,Dec from geocentric CIRS to ICRS astrometric.

      Given:
         ri,di  double  CIRS geocentric RA,Dec (radians)
         date1  double  TDB as a 2-part...
         date2  double  ...Julian Date (Note 1)

      Returned:
         rc,dc  double  ICRS astrometric RA,Dec (radians)
         eo     double  equation of the origins (ERA-GST, Note 4)

      Notes:

      1) The TDB date date1+date2 is a Julian Date, apportioned in any
         convenient way between the two arguments.  For example,
         JD(TDB)=2450123.7 could be expressed in any of these ways, among
         others:

                date1          date2

             2450123.7           0.0       (JD method)
             2451545.0       -1421.3       (J2000 method)
             2400000.5       50123.2       (MJD method)
             2450123.5           0.2       (date & time method)

         The JD method is the most natural and convenient to use in cases
         where the loss of several decimal digits of resolution is
         acceptable.  The J2000 method is best matched to the way the
         argument is handled internally and will deliver the optimum
         resolution.  The MJD method and the date & time methods are both
         good compromises between resolution and convenience.  For most
         applications of this function the choice will not be at all
         critical.

         TT can be used instead of TDB without any significant impact on
         accuracy.

      2) Iterative techniques are used for the aberration and light
         deflection corrections so that the functions eraAtic13 (or
         eraAticq) and eraAtci13 (or eraAtciq) are accurate inverses;
         even at the edge of the Sun's disk the discrepancy is only about
         1 nanoarcsecond.

      3) The available accuracy is better than 1 milliarcsecond, limited
         mainly by the precession-nutation model that is used, namely
         IAU 2000A/2006.  Very close to solar system bodies, additional
         errors of up to several milliarcseconds can occur because of
         unmodeled light deflection;  however, the Sun's contribution is
         taken into account, to first order.  The accuracy limitations of
         the ERFA function eraEpv00 (used to compute Earth position and
         velocity) can contribute aberration errors of up to
         5 microarcseconds.  Light deflection at the Sun's limb is
         uncertain at the 0.4 mas level.

      4) Should the transformation to (equinox based) J2000.0 mean place
         be required rather than (CIO based) ICRS coordinates, subtract the
         equation of the origins from the returned right ascension:
         RA = RI - EO.  (The eraAnp function can then be applied, as
         required, to keep the result in the conventional 0-2pi range.)

      Called:
         eraApci13    astrometry parameters, ICRS-CIRS, 2013
         eraAticq     quick CIRS to ICRS astrometric

      This revision:   2013 October 9

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.