xys00a

erfa.xys00a(date1, date2)[source]

For a given TT date, compute the X,Y coordinates of the Celestial Intermediate Pole and the CIO locator s, using the IAU 2000A precession-nutation model.

Parameters:
date1double array
date2double array
Returns:
xdouble array
ydouble array
sdouble array

Notes

Wraps ERFA function eraXys00a. The ERFA documentation is:

- - - - - - - - - -
 e r a X y s 0 0 a
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For a given TT date, compute the X,Y coordinates of the Celestial
Intermediate Pole and the CIO locator s, using the IAU 2000A
precession-nutation model.

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

Returned:
   x,y          double   Celestial Intermediate Pole (Note 2)
   s            double   the CIO locator s (Note 3)

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 Celestial Intermediate Pole coordinates are the x,y
   components of the unit vector in the Geocentric Celestial
   Reference System.

3) The CIO locator s (in radians) positions the Celestial
   Intermediate Origin on the equator of the CIP.

4) A faster, but slightly less accurate result (about 1 mas for
   X,Y), can be obtained by using instead the eraXys00b function.

Called:
   eraPnm00a    classical NPB matrix, IAU 2000A
   eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
   eraS00       the CIO locator s, given X,Y, IAU 2000A

Reference:

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

This revision:  2021 May 11

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