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eraGst06

rv = ERFA.gst06(uta, utb, tta, ttb, rnpb)

Greenwich apparent sidereal time, IAU 2006, given the NPB matrix.

Given:

   uta,utb  double        UT1 as a 2-part Julian Date (Notes 1,2)
   tta,ttb  double        TT as a 2-part Julian Date (Notes 1,2)
   rnpb     double[3][3]  nutation x precession x bias matrix

Returned (function value):

            double        Greenwich apparent sidereal time (radians)

Notes:

  1. The UT1 and TT dates uta+utb and tta+ttb respectively, are both Julian Dates, apportioned in any convenient way between the argument pairs. For example, JD(UT1)=2450123.7 could be expressed in any of these ways, among others:
           uta            utb

       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 (in the case of UT; the TT is not at all critical in this respect). The J2000 and MJD methods are good compromises between resolution and convenience. For UT, the date & time method is best matched to the algorithm that is used by the Earth rotation angle function, called internally: maximum precision is delivered when the uta argument is for 0hrs UT1 on the day in question and the utb argument lies in the range 0 to 1, or vice versa.

  1. Both UT1 and TT are required, UT1 to predict the Earth rotation and TT to predict the effects of precession-nutation. If UT1 is used for both purposes, errors of order 100 microarcseconds result.

  2. Although the function uses the IAU 2006 series for s+XY/2, it is otherwise independent of the precession-nutation model and can in practice be used with any equinox-based NPB matrix.

  3. The result is returned in the range 0 to 2pi.

Called:

   eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
   eraS06       the CIO locator s, given X,Y, IAU 2006
   eraAnp       normalize angle into range 0 to 2pi
   eraEra00     Earth rotation angle, IAU 2000
   eraEors      equation of the origins, given NPB matrix and s

Reference:

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.