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

#include "erfa.h"

int eraTdbtcb(double tdb1, double tdb2, double *tcb1, double *tcb2)
/*
**  - - - - - - - - - -
**   e r a T d b t c b
**  - - - - - - - - - -
**
**  Time scale transformation:  Barycentric Dynamical Time, TDB, to
**  Barycentric Coordinate Time, TCB.
**
**  Given:
**     tdb1,tdb2  double    TDB as a 2-part Julian Date
**
**  Returned:
**     tcb1,tcb2  double    TCB as a 2-part Julian Date
**
**  Returned (function value):
**                int       status:  0 = OK
**
**  Notes:
**
**  1) tdb1+tdb2 is Julian Date, apportioned in any convenient way
**     between the two arguments, for example where tdb1 is the Julian
**     Day Number and tdb2 is the fraction of a day.  The returned
**     tcb1,tcb2 follow suit.
**
**  2) The 2006 IAU General Assembly introduced a conventional linear
**     transformation between TDB and TCB.  This transformation
**     compensates for the drift between TCB and terrestrial time TT,
**     and keeps TDB approximately centered on TT.  Because the
**     relationship between TT and TCB depends on the adopted solar
**     system ephemeris, the degree of alignment between TDB and TT over
**     long intervals will vary according to which ephemeris is used.
**     Former definitions of TDB attempted to avoid this problem by
**     stipulating that TDB and TT should differ only by periodic
**     effects.  This is a good description of the nature of the
**     relationship but eluded precise mathematical formulation.  The
**     conventional linear relationship adopted in 2006 sidestepped
**     these difficulties whilst delivering a TDB that in practice was
**     consistent with values before that date.
**
**  3) TDB is essentially the same as Teph, the time argument for the
**     JPL solar system ephemerides.
**
**  Reference:
**
**     IAU 2006 Resolution B3
**
**  Copyright (C) 2013-2020, NumFOCUS Foundation.
**  Derived, with permission, from the SOFA library.  See notes at end of file.
*/
{

/* 1977 Jan 1 00:00:32.184 TT, as two-part JD */
   static const double t77td = ERFA_DJM0 + ERFA_DJM77;
   static const double t77tf = ERFA_TTMTAI/ERFA_DAYSEC;

/* TDB (days) at TAI 1977 Jan 1.0 */
   static const double tdb0 = ERFA_TDB0/ERFA_DAYSEC;

/* TDB to TCB rate */
   static const double elbb = ERFA_ELB/(1.0-ERFA_ELB);

   double d, f;


/* Result, preserving date format but safeguarding precision. */
   if ( fabs(tdb1) > fabs(tdb2) ) {
      d = t77td - tdb1;
      f  = tdb2 - tdb0;
      *tcb1 = tdb1;
      *tcb2 = f - ( d - ( f - t77tf ) ) * elbb;
   } else {
      d = t77td - tdb2;
      f  = tdb1 - tdb0;
      *tcb1 = f - ( d - ( f - t77tf ) ) * elbb;
      *tcb2 = tdb2;
   }

/* Status (always OK). */
   return 0;

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