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# pmpx

erfa.pmpx(rc, dc, pr, pd, px, rv, pmt, pob)

  Proper motion and parallax.

  Parameters:
    rc double array
    dc double array
    pr double array
    pd double array
    px double array
    rv double array
    pmt double array
    pob double array
  Returns:
    pco double array

  Notes

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

      - - - - - - - -
       e r a P m p x
      - - - - - - - -

      Proper motion and parallax.

      Given:
         rc,dc  double     ICRS RA,Dec at catalog epoch (radians)
         pr     double     RA proper motion (radians/year, Note 1)
         pd     double     Dec proper motion (radians/year)
         px     double     parallax (arcsec)
         rv     double     radial velocity (km/s, +ve if receding)
         pmt    double     proper motion time interval (SSB, Julian years)
         pob    double[3]  SSB to observer vector (au)

      Returned:
         pco    double[3]  coordinate direction (BCRS unit vector)

      Notes:

      1) The proper motion in RA is dRA/dt rather than cos(Dec)*dRA/dt.

      2) The proper motion time interval is for when the starlight
         reaches the solar system barycenter.

      3) To avoid the need for iteration, the Roemer effect (i.e. the
         small annual modulation of the proper motion coming from the
         changing light time) is applied approximately, using the
         direction of the star at the catalog epoch.

      References:

         1984 Astronomical Almanac, pp B39-B41.

         Urban, S. & Seidelmann, P. K. (eds), Explanatory Supplement to
         the Astronomical Almanac, 3rd ed., University Science Books
         (2013), Section 7.2.

      Called:
         eraPdp       scalar product of two p-vectors
         eraPn        decompose p-vector into modulus and direction

      This revision:   2021 April 3

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