obl80.c

#include "erfa.h"
#include "erfam.h"

double eraObl80(double date1, double date2)
/*
**  - - - - - - - - -
**   e r a O b l 8 0
**  - - - - - - - - -
**
**  Mean obliquity of the ecliptic, IAU 1980 model.
**
**  Given:
**     date1,date2   double    TT as a 2-part Julian Date (Note 1)
**
**  Returned (function value):
**                   double    obliquity of the ecliptic (radians, Note 2)
**
**  Notes:
**
**  1) The TT date date1+date2 is a Julian Date, apportioned in any
**     convenient way between the two arguments.  For example,
**     JD(TT)=2450123.7 could be expressed in any of these ways,
**     among others:
**
**            date1          date2
**
**         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 method is best matched to the way
**     the argument is handled internally and will deliver the
**     optimum resolution.  The MJD method and the date & time methods
**     are both good compromises between resolution and convenience.
**
**  2) The result is the angle between the ecliptic and mean equator of
**     date date1+date2.
**
**  Reference:
**
**     Explanatory Supplement to the Astronomical Almanac,
**     P. Kenneth Seidelmann (ed), University Science Books (1992),
**     Expression 3.222-1 (p114).
**
**  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 t, eps0;


/* Interval between fundamental epoch J2000.0 and given date (JC). */
   t = ((date1 - ERFA_DJ00) + date2) / ERFA_DJC;

/* Mean obliquity of date. */
   eps0 = ERFA_DAS2R * (84381.448  +
                  (-46.8150   +
                  (-0.00059   +
                  ( 0.001813) * t) * t) * t);

   return eps0;

/* Finished. */

}
/*----------------------------------------------------------------------
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