erfa.apci(date1, date2, ebpv, ehp, x, y, s)[source]

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

date1double array
date2double array
ebpvdouble array
ehpdouble array
xdouble array
ydouble array
sdouble array
astromeraASTROM array


Wraps ERFA function eraApci. The ERFA documentation is:

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 e r a A p c i
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For a terrestrial observer, prepare star-independent astrometry
parameters for transformations between ICRS and geocentric CIRS
coordinates.  The Earth ephemeris and CIP/CIO are supplied by the

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

   date1  double       TDB as a 2-part...
   date2  double       ...Julian Date (Note 1)
   ebpv   double[2][3] Earth barycentric position/velocity (au, au/day)
   ehp    double[3]    Earth heliocentric position (au)
   x,y    double       CIP X,Y (components of unit vector)
   s      double       the CIO locator s (radians)

   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 does not wish to provide the Earth
   ephemeris and CIP/CIO, the function eraApci13 can be used instead
   of the present function.  This computes the required quantities
   using other ERFA functions.

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*.

   eraApcg      astrometry parameters, ICRS-GCRS, geocenter
   eraC2ixys    celestial-to-intermediate matrix, given X,Y and s

This revision:   2013 September 25

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