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erfasrc/src/era00.c  view on Meta::CPAN

#include "erfa.h"

double eraEra00(double dj1, double dj2)
/*
**  - - - - - - - - -
**   e r a E r a 0 0
**  - - - - - - - - -
**
**  Earth rotation angle (IAU 2000 model).
**
**  Given:
**     dj1,dj2   double    UT1 as a 2-part Julian Date (see note)
**
**  Returned (function value):
**               double    Earth rotation angle (radians), range 0-2pi
**
**  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           0.0       (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.  The J2000 and MJD methods are good compromises
**     between resolution and convenience.  The date & time method is
**     best matched to the algorithm used:  maximum precision 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 adapted from Expression 22 of Capitaine et al.
**     2000.  The time argument has been expressed in days directly,
**     and, to retain precision, integer contributions have been
**     eliminated.  The same formulation is given in IERS Conventions
**     (2003), Chap. 5, Eq. 14.
**
**  Called:
**     eraAnp       normalize angle into range 0 to 2pi
**
**  References:
**
**     Capitaine N., Guinot B. and McCarthy D.D, 2000, Astron.
**     Astrophys., 355, 398-405.
**
**     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
**     IERS Technical Note No. 32, BKG (2004)
**
**  Copyright (C) 2013-2020, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{
   double d1, d2, t, f, theta;


/* Days since fundamental epoch. */
   if (dj1 < dj2) {
      d1 = dj1;
      d2 = dj2;
   } else {
      d1 = dj2;
      d2 = dj1;
   }
   t = d1 + (d2- ERFA_DJ00);

/* Fractional part of T (days). */
   f = fmod(d1, 1.0) + fmod(d2, 1.0);

/* Earth rotation angle at this UT1. */
   theta = eraAnp(ERFA_D2PI * (f + 0.7790572732640
                            + 0.00273781191135448 * t));

   return theta;

}
/*----------------------------------------------------------------------
**  
**  
**  Copyright (C) 2013-2020, 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