s00a¶

erfa.s00a(date1, date2)[source]¶

The CIO locator s, positioning the Celestial Intermediate Origin on the equator of the Celestial Intermediate Pole, using the IAU 2000A precession-nutation model.

Parameters:

date1double array
date2double array

Returns:

c_retvaldouble array

Notes

Wraps ERFA function eraS00a. The ERFA documentation is:

- - - - - - - -
 e r a S 0 0 a
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The CIO locator s, positioning the Celestial Intermediate Origin on
the equator of the Celestial Intermediate Pole, using the IAU 2000A
precession-nutation model.

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

Returned (function value):
                double    the CIO locator s in radians (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 CIO locator s is the difference between the right ascensions
   of the same point in two systems.  The two systems are the GCRS
   and the CIP,CIO, and the point is the ascending node of the
   CIP equator.  The CIO locator s remains a small fraction of
   1 arcsecond throughout 1900-2100.

3) The series used to compute s is in fact for s+XY/2, where X and Y
   are the x and y components of the CIP unit vector;  this series
   is more compact than a direct series for s would be.  The present
   function uses the full IAU 2000A nutation model when predicting
   the CIP position.  Faster results, with no significant loss of
   accuracy, can be obtained via the function eraS00b, which uses
   instead the IAU 2000B truncated model.

Called:
   eraPnm00a    classical NPB matrix, IAU 2000A
   eraBnp2xy    extract CIP X,Y from the BPN matrix
   eraS00       the CIO locator s, given X,Y, IAU 2000A

References:

   Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
   "Expressions for the Celestial Intermediate Pole and Celestial
   Ephemeris Origin consistent with the IAU 2000A precession-
   nutation model", Astron.Astrophys. 400, 1145-1154 (2003)

   n.b. The celestial ephemeris origin (CEO) was renamed "celestial
        intermediate origin" (CIO) by IAU 2006 Resolution 2.

   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.

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