era.xys00b.md

eraXys00b

[x, y, s] = ERFA.xys00b(date1, date2)

For a given TT date, compute the X,Y coordinates of the Celestial Intermediate Pole and the CIO locator s, using the IAU 2000B 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. The present function is faster, but slightly less accurate (about 1 mas in X,Y), than the eraXys00a function.

Called:

   eraPnm00b    classical NPB matrix, IAU 2000B
   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) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.