erfa.apcg13(date1, date2)[source]

For a geocentric observer, prepare star-independent astrometry parameters for transformations between ICRS and GCRS coordinates.

date1double array
date2double array
astromeraASTROM array


Wraps ERFA function eraApcg13. The ERFA documentation is:

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 e r a A p c g 1 3
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For a geocentric observer, prepare star-independent astrometry
parameters for transformations between ICRS and GCRS coordinates.
The caller supplies the date, and ERFA models are used to predict
the Earth ephemeris.

The parameters produced by this function are required in the
parallax, light deflection and aberration parts of the astrometric
transformation chain.

   date1  double     TDB as a 2-part...
   date2  double     ...Julian Date (Note 1)

   astrom eraASTROM* star-independent astrometry parameters:
    pmt    double       PM time interval (SSB, Julian years)
    eb     double[3]    SSB to observer (vector, au)
    eh     double[3]    Sun to observer (unit vector)
    em     double       distance from Sun to observer (au)
    v      double[3]    barycentric observer velocity (vector, c)
    bm1    double       sqrt(1-|v|^2): reciprocal of Lorenz factor
    bpn    double[3][3] bias-precession-nutation matrix
    along  double       unchanged
    xpl    double       unchanged
    ypl    double       unchanged
    sphi   double       unchanged
    cphi   double       unchanged
    diurab double       unchanged
    eral   double       unchanged
    refa   double       unchanged
    refb   double       unchanged


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

          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

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

2) All the vectors are with respect to BCRS axes.

3) In cases where the caller wishes to supply his own Earth
   ephemeris, the function eraApcg can be used instead of the present

4) This is one of several functions that inserts into the astrom
   structure star-independent parameters needed for the chain of
   astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed.

   The various functions support different classes of observer and
   portions of the transformation chain:

        functions         observer        transformation

     eraApcg eraApcg13    geocentric      ICRS <-> GCRS
     eraApci eraApci13    terrestrial     ICRS <-> CIRS
     eraApco eraApco13    terrestrial     ICRS <-> observed
     eraApcs eraApcs13    space           ICRS <-> GCRS
     eraAper eraAper13    terrestrial     update Earth rotation
     eraApio eraApio13    terrestrial     CIRS <-> observed

   Those with names ending in "13" use contemporary ERFA models to
   compute the various ephemerides.  The others accept ephemerides
   supplied by the caller.

   The transformation from ICRS to GCRS covers space motion,
   parallax, light deflection, and aberration.  From GCRS to CIRS
   comprises frame bias and precession-nutation.  From CIRS to
   observed takes account of Earth rotation, polar motion, diurnal
   aberration and parallax (unless subsumed into the ICRS <-> GCRS
   transformation), and atmospheric refraction.

5) The context structure astrom produced by this function is used by
   eraAtciq* and eraAticq*.

   eraEpv00     Earth position and velocity
   eraApcg      astrometry parameters, ICRS-GCRS, geocenter

This revision:   2013 October 9

Copyright (C) 2013-2023, NumFOCUS Foundation.
Derived, with permission, from the SOFA library.  See notes at end of file.