gst00a.c

#include "erfa.h"

double eraGst00a(double uta, double utb, double tta, double ttb)
/*
**  - - - - - - - - - -
**   e r a G s t 0 0 a
**  - - - - - - - - - -
**
**  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.
**
**  2) 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.
**
**  3) 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.
**
**  4) The result is returned in the range 0 to 2pi.
**
**  5) 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) 2013-2023, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   double gmst00, ee00a, gst;


   gmst00 = eraGmst00(uta, utb, tta, ttb);
   ee00a = eraEe00a(tta, ttb);
   gst = eraAnp(gmst00 + ee00a);

   return gst;

/* Finished. */

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2023, NumFOCUS Foundation.
**  All rights reserved.
**  
**  This library is derived, with permission, from the International
**  Astronomical Union's "Standards of Fundamental Astronomy" library,
**  available from http://www.iausofa.org.
**  
**  The ERFA version is intended to retain identical functionality to
**  the SOFA library, but made distinct through different function and
**  file names, as set out in the SOFA license conditions.  The SOFA
**  original has a role as a reference standard for the IAU and IERS,
**  and consequently redistribution is permitted only in its unaltered
**  state.  The ERFA version is not subject to this restriction and
**  therefore can be included in distributions which do not support the
**  concept of "read only" software.
**  
**  Although the intent is to replicate the SOFA API (other than
**  replacement of prefix names) and results (with the exception of
**  bugs;  any that are discovered will be fixed), SOFA is not
**  responsible for any errors found in this version of the library.
**  
**  If you wish to acknowledge the SOFA heritage, please acknowledge
**  that you are using a library derived from SOFA, rather than SOFA
**  itself.
**  
**  
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**  
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