#include "sofa.h"

double iauEra00(double dj1, double dj2)
/*
**  - - - - - - - - -
**   i a u E r a 0 0
**  - - - - - - - - -
**
**  Earth rotation angle (IAU 2000 model).
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards Of Fundamental Astronomy) software collection.
**
**  Status:  canonical 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:
**     iauAnp       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)
**
**  This revision:  2013 June 18
**
**  SOFA release 2016-05-03
**
**  Copyright (C) 2016 IAU SOFA Board.  See notes at end.
*/
{
   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- DJ00);

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

/* Earth rotation angle at this UT1. */
   theta = iauAnp(D2PI * (f + 0.7790572732640
                            + 0.00273781191135448 * t));

   return theta;

/*----------------------------------------------------------------------
**
**  Copyright (C) 2016
**  Standards Of Fundamental Astronomy Board
**  of the International Astronomical Union.
**
**  =====================
**  SOFA Software License
**  =====================
**
**  NOTICE TO USER:
**
**  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
**  CONDITIONS WHICH APPLY TO ITS USE.
**
**  1. The Software is owned by the IAU SOFA Board ("SOFA").
**
**  2. Permission is granted to anyone to use the SOFA software for any
**     purpose, including commercial applications, free of charge and
**     without payment of royalties, subject to the conditions and
**     restrictions listed below.
**
**  3. You (the user) may copy and distribute SOFA source code to others,
**     and use and adapt its code and algorithms in your own software,
**     on a world-wide, royalty-free basis.  That portion of your
**     distribution that does not consist of intact and unchanged copies
**     of SOFA source code files is a "derived work" that must comply
**     with the following requirements:
**
**     a) Your work shall be marked or carry a statement that it
**        (i) uses routines and computations derived by you from
**        software provided by SOFA under license to you; and
**        (ii) does not itself constitute software provided by and/or
**        endorsed by SOFA.
**
**     b) The source code of your derived work must contain descriptions
**        of how the derived work is based upon, contains and/or differs
**        from the original SOFA software.
**
**     c) The names of all routines in your derived work shall not
**        include the prefix "iau" or "sofa" or trivial modifications
**        thereof such as changes of case.
**
**     d) The origin of the SOFA components of your derived work must
**        not be misrepresented;  you must not claim that you wrote the
**        original software, nor file a patent application for SOFA
**        software or algorithms embedded in the SOFA software.
**
**     e) These requirements must be reproduced intact in any source
**        distribution and shall apply to anyone to whom you have
**        granted a further right to modify the source code of your
**        derived work.
**
**     Note that, as originally distributed, the SOFA software is
**     intended to be a definitive implementation of the IAU standards,
**     and consequently third-party modifications are discouraged.  All
**     variations, no matter how minor, must be explicitly marked as
**     such, as explained above.
**
**  4. You shall not cause the SOFA software to be brought into
**     disrepute, either by misuse, or use for inappropriate tasks, or
**     by inappropriate modification.
**
**  5. The SOFA software is provided "as is" and SOFA makes no warranty
**     as to its use or performance.   SOFA does not and cannot warrant
**     the performance or results which the user may obtain by using the
**     SOFA software.  SOFA makes no warranties, express or implied, as
**     to non-infringement of third party rights, merchantability, or
**     fitness for any particular purpose.  In no event will SOFA be
**     liable to the user for any consequential, incidental, or special
**     damages, including any lost profits or lost savings, even if a
**     SOFA representative has been advised of such damages, or for any
**     claim by any third party.
**
**  6. The provision of any version of the SOFA software under the terms
**     and conditions specified herein does not imply that future
**     versions will also be made available under the same terms and
**     conditions.
*
**  In any published work or commercial product which uses the SOFA
**  software directly, acknowledgement (see www.iausofa.org) is
**  appreciated.
**
**  Correspondence concerning SOFA software should be addressed as
**  follows:
**
**      By email:  sofa@ukho.gov.uk
**      By post:   IAU SOFA Center
**                 HM Nautical Almanac Office
**                 UK Hydrographic Office
**                 Admiralty Way, Taunton
**                 Somerset, TA1 2DN
**                 United Kingdom
**
**--------------------------------------------------------------------*/
}