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eraAtic13

[rc, dc, eo] = ERFA.atic13(ri, di, date1, date2)

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.

  1. 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.

  2. 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.

  3. 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.