pro precess_xyz,x,y,z,equinox1,equinox2
;+
; 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
;       EQUINOX2: 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.
;
;EXAMPLE:
;	Precess 1950 equinox coords x, y and z to 2000.
;	IDL> precess_xyz,x,y,z, 1950, 2000
;
;HISTORY:
;	Written by P. Plait/ACC March 24 1999 
;	   (unit vectors provided by D. Lindler)
;       Use /Radian call to PRECESS     W. Landsman     November 2000
;       Use two parameter call to ATAN   W. Landsman    June 2001
;-
;check inputs
   if N_params() NE 5 then begin
      print,'Syntax - PRECESS_XYZ,x,y,z,equinox1,equinox2'
      return
   endif

;take input coords and convert to ra and dec (in radians)

   ra = atan(y,x)
   del = sqrt(x*x + y*y + z*z)  ;magnitude of distance to Sun
   dec = asin(z/del) 

;   precess the ra and dec
    precess, ra, dec, equinox1, equinox2, /Radian

;convert back to x, y, z
   xunit = cos(ra)*cos(dec)
   yunit = sin(ra)*cos(dec)
   zunit = sin(dec)

   x = xunit * del
   y = yunit * del
   z = zunit * del

   return
   end