Procedure

   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 vacuo (km/sec).

Detailed_Input

   None.

Detailed_Output

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

Parameters

 
   None. 
 

Exceptions

   Error free.

Files

   None.

Particulars

   The function always returns the constant value shown above.

Examples

   The following example uses clight_c to determine the one-way
   light-time (tau) to an object whose position relative to an
   observer is contained in pos.

      tau = vnorm_c ( pos ) / clight_c ();

   Note that the SPK readers

      spkezr_c 
      spkez_c 
      spkpos_c 
      spkezp_c 

   return the one-way light time as an output, for example

      spkez_c  ( ..., &tau );

Restrictions

   None.

Literature_References

   None.

Author_and_Institution

   N.J. Bachman    (JPL)
   W.L. Taber      (JPL)
   I.M. Underwood  (JPL)
   E.D. Wright     (JPL)

Version

   -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