Index of Functions: A B C D E F G H I J K L M N O P Q R S T U V W X
Index Page clight_c

Table of contents

Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       clight_c ( C, Speed of light in a vacuum )

       SpiceDouble clight_c ( void )

#### Abstract

       Return the speed of light in a vacuum (IAU official
       value, in km/sec).

#### Required_Reading

       None.

#### Keywords

       CONSTANTS

#### Brief_I/O

       The function returns the speed of light in vacuum (km/sec).

#### Detailed_Input

       None.

#### Detailed_Output

       The function returns the IAU official value for the speed of light
       in vacuum: 299792.458 km/sec.

#### Parameters

       None.

#### Exceptions

       Error free.

#### Files

       None.

#### Particulars

       The function always returns the constant value shown above.

#### 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 IAU official double precision value
          for the constant speed of light in the vacuum, in km/sec.

          Example code begins here.

          /.
             Program clight_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main( )
          {
             /.
             Display the value for the constant speed of light in the
             vacuum.
             ./
             printf ( "Speed of light: %12.4f\n", clight_c( ) );

             return ( 0 );
          }

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

          Speed of light:  299792.4580

       2) The following example uses clight_c to determine the one-way
          light-time (tau) to Mars whose geometric position relative to
          the Earth is available.

          Use the SPK kernel below to load the required ephemerides
          for Mars and the Earth.

             de405.bsp

          Example code begins here.

          /.
             Program clight_ex2
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main( )
          {
             /.
             Local constants
             ./
             #define  ET             0.0

             /.
             Local variables
             ./
             SpiceDouble             lt;
             SpiceDouble             pos    [ 3 ];
             SpiceDouble             tau;

             /.
             Load the SPK file.
             ./
             furnsh_c ( "de405.bsp" );

             /.
             Obtain the geometric position of Mars relative to the Earth at
             the given ET, in J2000 frame.
             ./
             spkpos_c ( "MARS", ET, "J2000", "NONE", "EARTH", pos, &lt );

             /.
             Compute the one-way light time using clight_c
             ./
             tau = vnorm_c ( pos ) / clight_c ( );

             /.
             Display the one-way light time values obtained from spkpos_c
             and using clight_c.
             ./
             printf ( "One-way light time (spkpos_c): %16.9f\n", lt );
             printf ( "One-way light time (clight_c): %16.9f\n", tau  );

             return ( 0 );
          }

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

          One-way light time (spkpos_c):    922.961207764
          One-way light time (clight_c):    922.961207764

          Note that the following SPK readers

             spkezr_c
             spkez_c
             spkpos_c
             spkezp_c

          return the one-way light time as an output.

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       E.D. Wright         (JPL)

#### Version

       -CSPICE Version 1.0.3, 13-AUG-2021 (JDR)

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

       -CSPICE Version 1.0.2, 07-FEB-2008 (EDW) (NJB)

           Corrected typos in header titles:

           Detailed Input to -Detailed_Input
           Detailed Output to -Detailed_Output

           Updated example to show pointer output argument
           `tau' and list other high-level SPK routines that
           return light time. Call to bodmat_c was removed
           from example.

       -CSPICE Version 1.0.1, 11-NOV-2006 (EDW)

           Added -Parameters section header.

       -CSPICE Version 1.0.0, 16-APR-1999 (EDW)

#### Index_Entries

       measured velocity of light in a vacuum

#### Link to routine clight_c source file clight_c.c

    Fri Dec 31 18:41:03 2021