Procedure

   b1950_c ( Besselian Date 1950.0 ) 

   SpiceDouble b1950_c ( void )

Abstract

   Return the Julian Date corresponding to Besselian Date 1950.0.

Required_Reading

   None.

Keywords

   CONSTANTS

Brief_I/O

   The function returns the Julian Date corresponding to Besselian
   date 1950.0.

Detailed_Input

   None.

Detailed_Output

   The function returns 2433282.42345905, the Julian Date corresponding
   to Besselian Date 1950.0 as reported by Lieske [1].

Parameters

   None.

Exceptions

   Error free.

Files

   None.

Particulars

   Lieske [1] defines a mapping from Julian Ephemeris Date
   to Besselian:

      BE = 1900. + (JED - 2415020.31352)/365.242198781

   The inverse mapping being:

      JED = (BE - 1900.) * 365.242198781 + 2415020.31352

Examples

   The numerical results shown for these examples may differ across
   platforms. The results depend on the SPICE kernels used as
   input, the compiler and supporting libraries, and the machine
   specific arithmetic implementation.

   1) Display the double precision value for the Julian Date
      corresponding to the Besselian date 1950.0.

      Example code begins here.


      /.
         Program b1950_ex1
      ./
      #include <stdio.h>
      #include "SpiceUsr.h"

      int main( )
      {
         /.
         Display the B1950 date in 16.8 floating point format
         ./
         printf ( "B1950 date: %16.8f\n", b1950_c() );

         return ( 0 );
      }


      When this program was executed on a Mac/Intel/cc/64-bit
      platform, the output was:


      B1950 date: 2433282.42345905


   2) Convert an input time in UTC format to TDB seconds past the
      following reference epochs:

         - Besselian date 1900 and 1950; and

         - Julian date 1900, 1950, 2000 and 2100.

      Use the LSK kernel below to load the leap seconds and time
      constants required for the conversions.

         naif0012.tls


      Example code begins here.


      /.
         Program b1950_ex2
      ./
      #include <stdio.h>
      #include "SpiceUsr.h"

      int main( )
      {
         /.
         Local constants.
         ./
         #define        UTCSTR        "1991-NOV-26"

         /.
         Local variables.
         ./
         SpiceDouble             et;
         SpiceDouble             jed;

         /.
         Load the LSK file.
         ./
         furnsh_c ( "naif0012.tls" );

         /.
         Convert input UTC string to Ephemeris Time.
         ./
         str2et_c ( UTCSTR, &et );
         printf ( "Input ephemeris time  : %20.3f\n\n", et );

         /.
         Convert the Ephemeris Time to Julian ephemeris date, i.e.
         Julian date relative to TDB time scale.
         ./
         jed = unitim_c ( et, "ET", "JED" );

         /.
         Convert Julian Date to TDB seconds past the reference epochs
         and output the results.
         ./
         printf ( "TDB seconds past B1900: %20.3f\n",
                  ( jed - b1900_c() ) * spd_c()     );
         printf ( "TDB seconds past B1950: %20.3f\n",
                  ( jed - b1950_c() ) * spd_c()     );
         printf ( "TDB seconds past J1900: %20.3f\n",
                  ( jed - j1900_c() ) * spd_c()     );
         printf ( "TDB seconds past J1950: %20.3f\n",
                  ( jed - j1950_c() ) * spd_c()     );
         printf ( "TDB seconds past J2000: %20.3f\n",
                  ( jed - j2000_c() ) * spd_c()     );
         printf ( "TDB seconds past J2100: %20.3f\n",
                  ( jed - j2100_c() ) * spd_c()     );

         return ( 0 );
      }


      When this program was executed on a Mac/Intel/cc/64-bit
      platform, the output was:


      Input ephemeris time  :       -255614341.817

      TDB seconds past B1900:       2900118570.055
      TDB seconds past B1950:       1322272271.321
      TDB seconds past J1900:       2900145658.183
      TDB seconds past J1950:       1322265658.183
      TDB seconds past J2000:       -255614341.817
      TDB seconds past J2100:      -3411374341.817

Restrictions

   None.

Literature_References

   [1]  J. Lieske, "Precession Matrix Based on IAU (1976) System of
        Astronomical Constants," Astron. Astrophys. 73, 282-284,
        1979.

Author_and_Institution

   J. Diaz del Rio     (ODC Space)
   W.L. Taber          (JPL)
   I.M. Underwood      (JPL)
   E.D. Wright         (JPL)

Version

   -CSPICE Version 2.0.1, 06-JUL-2021 (JDR)

       Edited the header to comply with NAIF standard. Added
       complete code examples.

   -CSPICE Version 2.0.0, 01-SEP-2005 (EDW)

       This routine now returns the value reported in the Lieske
       paper:

          2433282.42345905

       The same value returned by the FORTRAN SPICELIB routine
       B1950.

       This routine previously returned the value reported in the
       "Explanatory Supplement to the Astronomical Almanac", 1992,
       page 699:

          2433282.423

       The ESAA value describing a truncation of the Lieske value.
       The difference between the two values expressed as seconds
       yields approximately 39.662 seconds.

   -CSPICE Version 1.0.1, 08-FEB-1998 (EDW)

       Corrected and clarified header entries.

   -CSPICE Version 1.0.0, 25-OCT-1997 (EDW) (WLT) (IMU)

Index_Entries

   besselian date 1950.0