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# aper13

erfa.aper13(ut11, ut12, astrom)

  In the star-independent astrometry parameters, update only the Earth rotation angle.

  Parameters
    ut11 double array
    ut12 double array
    astrom eraASTROM array
  Returns
    astrom eraASTROM array

  Notes

  Wraps ERFA function `eraAper13`. Note that, unlike the erfa routine, the python wrapper does not change astrom in-place. The ERFA documentation is:

      - - - - - - - - - -
       e r a A p e r 1 3
      - - - - - - - - - -

      In the star-independent astrometry parameters, update only the
      Earth rotation angle.  The caller provides UT1, (n.b. not UTC).

      Given:
         ut11    double      UT1 as a 2-part...
         ut12    double      ...Julian Date (Note 1)
         astrom  eraASTROM        star-independent astrometry parameters:
          pmt    double       not used
          eb     double[3]    not used
          eh     double[3]    not used
          em     double       not used
          v      double[3]    not used
          bm1    double       not used
          bpn    double[3][3] not used
          along  double       longitude + s' (radians)
          xpl    double       not used
          ypl    double       not used
          sphi   double       not used
          cphi   double       not used
          diurab double       not used
          eral   double       not used
          refa   double       not used
          refb   double       not used

      Returned:
         astrom  eraASTROM        star-independent astrometry parameters:
          pmt    double       unchanged
          eb     double[3]    unchanged
          eh     double[3]    unchanged
          em     double       unchanged
          v      double[3]    unchanged
          bm1    double       unchanged
          bpn    double[3][3] unchanged
          along  double       unchanged
          xpl    double       unchanged
          ypl    double       unchanged
          sphi   double       unchanged
          cphi   double       unchanged
          diurab double       unchanged
          eral   double       "local" Earth rotation angle (radians)
          refa   double       unchanged
          refb   double       unchanged

      Notes:

      1) The UT1 date (n.b. not UTC) ut11+ut12 is a Julian Date,
         apportioned in any convenient way between the arguments ut11 and
         ut12.  For example, JD(UT1)=2450123.7 could be expressed in any
         of these ways, among others:

                ut11           ut12

             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 and MJD methods are good compromises
         between resolution and convenience.  The date & time method is
         best matched to the algorithm used:  maximum precision is
         delivered when the ut11 argument is for 0hrs UT1 on the day in
         question and the ut12 argument lies in the range 0 to 1, or vice
         versa.

      2) If the caller wishes to provide the Earth rotation angle itself,
         the function eraAper can be used instead.  One use of this
         technique is to substitute Greenwich apparent sidereal time and
         thereby to support equinox based transformations directly.

      3) 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.

      Called:
         eraAper      astrometry parameters: update ERA
         eraEra00     Earth rotation angle, IAU 2000

      This revision:   2013 September 25

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