Procedure

   deltet_c ( Delta ET, ET - UTC ) 

   void deltet_c ( SpiceDouble      epoch,
                   ConstSpiceChar * eptype,
                   SpiceDouble    * delta )

Abstract

   Return the value of Delta ET (ET-UTC) for an input epoch.

Required_Reading

   TIME
   KERNEL

Keywords

   TIME

Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   epoch      I   Input epoch (seconds past J2000).
   eptype     I   Type of input epoch ("UTC" or "ET").
   delta      O   Delta ET (ET-UTC) at input epoch.

Detailed_Input

   epoch       is the epoch at which "delta ET" is to be computed.
               `epoch' may be either UTC or ephemeris seconds past
               J2000, as specified by `eptype'.

   eptype      is the type of input epoch. It may be either
               of the following:

                  "UTC"    UTC seconds past J2000 UTC.

                  "ET"     Ephemeris seconds past J2000 TDB,
                           also known as barycentric dynamical
                           time (TDB).

Detailed_Output

   delta       is the value of

                  "delta ET" = ET - UTC

               at the input epoch. This is added to UTC to give
               ET, or subtracted from ET to give UTC. The routine
               is reversible: that is, given the following calls,

                  deltet_c ( utc,      "UTC", &del1 );
                  deltet_c ( utc+del1, "ET",  &del2 );

               the expression

                  ( del1 == del2 )

               is always SPICETRUE.

Parameters

   None.

Exceptions

   1)  If the input epoch is not recognized, the error
       SPICE(INVALIDEPOCH) is signaled by a routine in the call tree
       of this routine.

   2)  If the variables necessary for the computation of `delta' have
       not been loaded into the kernel pool, the error
       SPICE(KERNELVARNOTFOUND) is signaled by a routine in the call
       tree of this routine.

   3)  If the number of leapseconds in the pool is greater than the
       local leapseconds buffer size, the error SPICE(BUFFEROVERFLOW)
       is signaled by a routine in the call tree of this routine.

   4)  If the `eptype' input string pointer is null, the error
       SPICE(NULLPOINTER) is signaled.

   5)  If the `eptype' input string has zero length, the error
       SPICE(EMPTYSTRING) is signaled.

Files

   None.

Particulars

   The value of Delta ET is given by

      delta = ( ET - TAI ) + leap seconds

   where TAI is the atomic time corresponding to the input epoch.

Examples

   The following example shows how deltet_c may be used to convert
   from UTC seconds past J2000 to TDB seconds past J2000.

      deltet_c ( utcsec, "UTC", &delta );
      et = utcsec + delta

   The following example shows how deltet_c may be used to convert
   from ephemeris seconds past J2000 to UTC seconds past J2000.

      deltet_c ( et, "et", &delta );
      utcsec = et - delta;

   See the TIME Required Reading for further examples.

Restrictions

   1)  The routines str2et_c and et2utc_c are preferred for conversions
       between UTC and ET. This routine is provided mainly as a
       utility for str2et_c and et2utc_c.

   2)  A leapseconds kernel containing leapseconds and relativistic
       terms MUST be loaded prior to calling this function.
       Examples demonstrating how to load a kernel pool are included
       in the Required Reading file time.req and in the -Examples
       section of this header. For more general information about
       kernel pools, please consult the Required Reading file
       kernel.req.

Literature_References

   [1]  "The Astronomical Almanac for the Year 1990," United States
        Naval Observatory, U.S. Government Printing Office,
        Washington, D.C., 1989.

Author_and_Institution

   N.J. Bachman        (JPL)
   J. Diaz del Rio     (ODC Space)
   W.M. Owen           (JPL)
   I.M. Underwood      (JPL)

Version

   -CSPICE Version 1.0.1, 24-AUG-2021 (JDR)

       Edited the header to comply with NAIF standard. Added entry #2
       to -Restrictions section.

   -CSPICE Version 1.0.0, 01-AUG-2003 (NJB) (WMO) (IMU)

Index_Entries

   difference between ephemeris time and utc

Link to routine deltet_c source file deltet_c.c