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eraGst00a

rv = ERFA.gst00a(uta, utb, tta, ttb)

Greenwich apparent sidereal time (consistent with IAU 2000 resolutions).

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)

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. This GAST is compatible with the IAU 2000 resolutions and must be used only in conjunction with other IAU 2000 compatible components such as precession-nutation.

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

  4. The algorithm is from Capitaine et al. (2003) and IERS Conventions 2003.

Called:

   eraGmst00    Greenwich mean sidereal time, IAU 2000
   eraEe00a     equation of the equinoxes, IAU 2000A
   eraAnp       normalize angle into range 0 to 2pi

References:

Capitaine, N., Wallace, P.T. and McCarthy, D.D., "Expressions to implement the IAU 2000 definition of UT1", Astronomy & Astrophysics, 406, 1135-1149 (2003)

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.