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erfasrc/src/eect00.c  view on Meta::CPAN

#include "erfa.h"

double eraEect00(double date1, double date2)
/*
**  - - - - - - - - - -
**   e r a E e c t 0 0
**  - - - - - - - - - -
**
**  Equation of the equinoxes complementary terms, consistent with
**  IAU 2000 resolutions.
**
**  Given:
**     date1,date2  double   TT as a 2-part Julian Date (Note 1)
**
**  Returned (function value):
**                  double   complementary terms (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 "complementary terms" are part of the equation of the
**     equinoxes (EE), classically the difference between apparent and
**     mean Sidereal Time:
**
**        GAST = GMST + EE
**
**     with:
**
**        EE = dpsi * cos(eps)
**
**     where dpsi is the nutation in longitude and eps is the obliquity
**     of date.  However, if the rotation of the Earth were constant in
**     an inertial frame the classical formulation would lead to
**     apparent irregularities in the UT1 timescale traceable to side-
**     effects of precession-nutation.  In order to eliminate these
**     effects from UT1, "complementary terms" were introduced in 1994
**     (IAU, 1994) and took effect from 1997 (Capitaine and Gontier,
**     1993):
**
**        GAST = GMST + CT + EE
**
**     By convention, the complementary terms are included as part of
**     the equation of the equinoxes rather than as part of the mean
**     Sidereal Time.  This slightly compromises the "geometrical"
**     interpretation of mean sidereal time but is otherwise
**     inconsequential.
**
**     The present function computes CT in the above expression,
**     compatible with IAU 2000 resolutions (Capitaine et al., 2002, and
**     IERS Conventions 2003).
**
**  Called:
**     eraFal03     mean anomaly of the Moon
**     eraFalp03    mean anomaly of the Sun
**     eraFaf03     mean argument of the latitude of the Moon
**     eraFad03     mean elongation of the Moon from the Sun
**     eraFaom03    mean longitude of the Moon's ascending node
**     eraFave03    mean longitude of Venus
**     eraFae03     mean longitude of Earth
**     eraFapa03    general accumulated precession in longitude
**
**  References:
**
**     Capitaine, N. & Gontier, A.-M., Astron.Astrophys., 275,
**     645-650 (1993)
**
**     Capitaine, N., Wallace, P.T. and McCarthy, D.D., "Expressions to
**     implement the IAU 2000 definition of UT1", Astron.Astrophys., 406,
**     1135-1149 (2003)
**
**     IAU Resolution C7, Recommendation 3 (1994)
**
**     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.
*/
{
/* Time since J2000.0, in Julian centuries */
   double t;

/* Miscellaneous */
   int i, j;
   double a, s0, s1;

/* Fundamental arguments */
   double fa[14];

/* Returned value. */
   double eect;

/* ----------------------------------------- */