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Diffstat (limited to 'ast/erfa/gmst82.c')
-rw-r--r-- | ast/erfa/gmst82.c | 160 |
1 files changed, 160 insertions, 0 deletions
diff --git a/ast/erfa/gmst82.c b/ast/erfa/gmst82.c new file mode 100644 index 0000000..9484b66 --- /dev/null +++ b/ast/erfa/gmst82.c @@ -0,0 +1,160 @@ +#include "erfa.h" + +double eraGmst82(double dj1, double dj2) +/* +** - - - - - - - - - - +** e r a G m s t 8 2 +** - - - - - - - - - - +** +** 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. +** +** 2) 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. +** +** 3) 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). +** +** Copyright (C) 2013-2016, NumFOCUS Foundation. +** Derived, with permission, from the SOFA library. See notes at end of file. +*/ +{ +/* Coefficients of IAU 1982 GMST-UT1 model */ + double A = 24110.54841 - ERFA_DAYSEC / 2.0; + double B = 8640184.812866; + double C = 0.093104; + double D = -6.2e-6; + +/* Note: the first constant, A, has to be adjusted by 12 hours */ +/* because the UT1 is supplied as a Julian date, which begins */ +/* at noon. */ + + double d1, d2, t, f, gmst; + + +/* Julian centuries since fundamental epoch. */ + if (dj1 < dj2) { + d1 = dj1; + d2 = dj2; + } else { + d1 = dj2; + d2 = dj1; + } + t = (d1 + (d2 - ERFA_DJ00)) / ERFA_DJC; + +/* Fractional part of JD(UT1), in seconds. */ + f = ERFA_DAYSEC * (fmod(d1, 1.0) + fmod(d2, 1.0)); + +/* GMST at this UT1. */ + gmst = eraAnp(ERFA_DS2R * ((A + (B + (C + D * t) * t) * t) + f)); + + return gmst; + +} +/*---------------------------------------------------------------------- +** +** +** Copyright (C) 2013-2016, 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. +** +** +** TERMS AND CONDITIONS +** +** Redistribution and use in source and binary forms, with or without +** modification, are permitted provided that the following conditions +** are met: +** +** 1 Redistributions of source code must retain the above copyright +** notice, this list of conditions and the following disclaimer. +** +** 2 Redistributions in binary form must reproduce the above copyright +** notice, this list of conditions and the following disclaimer in +** the documentation and/or other materials provided with the +** distribution. +** +** 3 Neither the name of the Standards Of Fundamental Astronomy Board, +** the International Astronomical Union nor the names of its +** contributors may be used to endorse or promote products derived +** from this software without specific prior written permission. +** +** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS +** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT +** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS +** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE +** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, +** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, +** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; +** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER +** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN +** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE +** POSSIBILITY OF SUCH DAMAGE. +** +*/ |