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eraGmst82

rv = ERFA.gmst82(dj1, dj2)

Universal Time to Greenwich mean sidereal time (IAU 1982 model).

Given:

   dj1,dj2    double    UT1 Julian Date (see note)

Returned (function value):

              double    Greenwich mean sidereal time (radians)

Notes:

  1. The UT1 date dj1+dj2 is a Julian Date, apportioned in any convenient way between the arguments dj1 and dj2. For example, JD(UT1)=2450123.7 could be expressed in any of these ways, among others:
           dj1            dj2

       2450123.7          0          (JD method)
        2451545        -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 and MJD methods are good compromises between resolution and convenience. The date & time method is best matched to the algorithm used: maximum accuracy (or, at least, minimum noise) is delivered when the dj1 argument is for 0hrs UT1 on the day in question and the dj2 argument lies in the range 0 to 1, or vice versa.

  1. The algorithm is based on the IAU 1982 expression. This is always described as giving the GMST at 0 hours UT1. In fact, it gives the difference between the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of ST with respect to UT. When whole days are ignored, the expression happens to equal the GMST at 0 hours UT1 each day.

  2. In this function, the entire UT1 (the sum of the two arguments dj1 and dj2) is used directly as the argument for the standard formula, the constant term of which is adjusted by 12 hours to take account of the noon phasing of Julian Date. The UT1 is then added, but omitting whole days to conserve accuracy.

Called:

   eraAnp       normalize angle into range 0 to 2pi

References:

Transactions of the International Astronomical Union, XVIII B, 67 (1983).

Aoki et al., Astron.Astrophys., 105, 359-361 (1982).

This revision: 2021 May 11

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.