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# xy06

erfa.xy06(date1, date2)

  X,Y coordinates of celestial intermediate pole from series based on IAU 2006 precession and IAU 2000A nutation.

  Parameters
    date1 double array
    date2 double array
  Returns
    x double array
    y double array

  Notes

  Wraps ERFA function `eraXy06`. The ERFA documentation is:

      - - - - - - - -
       e r a X y 0 6
      - - - - - - - -

      X,Y coordinates of celestial intermediate pole from series based
      on IAU 2006 precession and IAU 2000A nutation.

      Given:
         date1,date2  double     TT as a 2-part Julian Date (Note 1)

      Returned:
         x,y          double     CIP X,Y coordinates (Note 2)

      Notes:

      1) The TT date date1+date2 is a Julian Date, apportioned in any
         convenient way between the two arguments.  For example,
         JD(TT)=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.

      2) The X,Y coordinates are those of the unit vector towards the
         celestial intermediate pole.  They represent the combined effects
         of frame bias, precession and nutation.

      3) The fundamental arguments used are as adopted in IERS Conventions
         (2003) and are from Simon et al. (1994) and Souchay et al.
         (1999).

      4) This is an alternative to the angles-based method, via the ERFA
         function eraFw2xy and as used in eraXys06a for example.  The two
         methods agree at the 1 microarcsecond level (at present), a
         negligible amount compared with the intrinsic accuracy of the
         models.  However, it would be unwise to mix the two methods
         (angles-based and series-based) in a single application.

      Called:
         eraFal03     mean anomaly of the Moon
         eraFalp03    mean anomaly of the Sun
         eraFaf03     mean argument of the latitude of the Moon
         eraFad03     mean elongation of the Moon from the Sun
         eraFaom03    mean longitude of the Moon's ascending node
         eraFame03    mean longitude of Mercury
         eraFave03    mean longitude of Venus
         eraFae03     mean longitude of Earth
         eraFama03    mean longitude of Mars
         eraFaju03    mean longitude of Jupiter
         eraFasa03    mean longitude of Saturn
         eraFaur03    mean longitude of Uranus
         eraFane03    mean longitude of Neptune
         eraFapa03    general accumulated precession in longitude

      References:

         Capitaine, N., Wallace, P.T. & Chapront, J., 2003,
         Astron.Astrophys., 412, 567

         Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855

         McCarthy, D. D., Petit, G. (eds.), 2004, IERS Conventions (2003),
         IERS Technical Note No. 32, BKG

         Simon, J.L., Bretagnon, P., Chapront, J., Chapront-Touze, M.,
         Francou, G. & Laskar, J., Astron.Astrophys., 1994, 282, 663

         Souchay, J., Loysel, B., Kinoshita, H., Folgueira, M., 1999,
         Astron.Astrophys.Supp.Ser. 135, 111

         Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981

      This revision:  2021 May 11

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