idlastro / Astronomical Utilities: PRECESS_XYZ

[Source code]

NAME
PRECESS_XYZ
PURPOSE
Precess equatorial geocentric rectangular coordinates. 
CALLING SEQUENCE
precess_xyz, x, y, z, equinox1, equinox2
INPUT/OUTPUT
x,y,z: scalars or vectors giving heliocentric rectangular coordinates
      THESE ARE CHANGED UPON RETURNING.
INPUT
EQUINOX1: equinox of input coordinates, numeric scalar
QUINOX2: equinox of output coordinates, numeric scalar
OUTPUT
x,y,z are changed upon return
NOTES
The equatorial geocentric rectangular coords are converted
   to RA and Dec, precessed in the normal way, then changed
   back to x, y and z using unit vectors.
XAMPLE
Precess 1950 equinox coords x, y and z to 2000.
IDL> precess_xyz,x,y,z, 1950, 2000
ISTORY
Written by P. Plait/ACC March 24 1999 
   (unit vectors provided by D. Lindler)
se /Radian call to PRECESS     W. Landsman     November 2000
se two parameter call to ATAN   W. Landsman    June 2001