source: trunk/MagicSoft/slalib/rverot.c@ 18403

#include "slalib.h"
#include "slamac.h"
float slaRverot ( float phi, float ra, float da, float st )
/*
**  - - - - - - - - - -
**   s l a R v e r o t
**  - - - - - - - - - -
**
**  Velocity component in a given direction due to Earth rotation.
**
**  (single precision)
**
**  Given:
**     phi     float    latitude of observing station (geodetic)
**     ra,da   float    apparent RA,Dec
**     st      float    local apparent sidereal time
**
**     phi, ra, dec and st are all in radians.
**
**  Result:
**     Component of Earth rotation in direction ra,da (km/s)
**
**  Sign convention:
**     The result is +ve when the observer is receding from the
**     given point on the sky.
**
**  Accuracy:
**     The simple algorithm used assumes a spherical Earth, of
**     a radius chosen to give results accurate to about 0.0005 km/s
**     for observing stations at typical latitudes and heights.  For
**     applications requiring greater precision, use the routine
**     slaPvobs.
**
**  Last revision:   9 April 1998
**
**  Copyright P.T.Wallace.  All rights reserved.
*/

#define ESPEED 0.4655   /* Nominal mean sidereal speed of Earth equator
                           in km/s (the actual value is about 0.4651) */

{
  return (float) ( ESPEED * cos ( (double) phi ) *
                            sin ( (double) ( st - ra ) ) *
                            cos ( (double) da ) );
}