function constellation, epoch, ra, dec
;
; Given a Julian Epoch (Gregorian year & fraction), and the Right 
; Ascension (hours) and Declination of a point on the celestial
; sphere, this function returns the official IAU abbreviation of
; the name of the constellation in which the point lies.
;
; B. Knapp, 2001-04-26
;
; RCS Data:
;
; $Header$
;
; $Log$
;
; Print usage?
  n = n_elements( epoch )
  if n eq 0 or n ne n_elements(ra) or n ne n_elements(dec) then begin
     print, ' '
     print, ' Given a Julian Epoch (Gregorian year & fraction), and the'
     print, ' Right Ascension (hours) and Declination (degrees) of a point'
     print, ' on the celestial sphere, this function returns the official'
     print, ' IAU 3-letter abbreviation for the name of the constellation'
     print, ' in which the point lies. (Scalars or same-length arrays are'
     print, ' accepted.)'
     print, ' '
     print, ' Usage:'
     print, ' '
     print, '     iau_abbrev = constellation( epoch, ra, dec )'
     return, ' '
  endif
;
; Use 32-bit or 64-bit shareable object library?
  mbits = ((!version.release ge 5.4) ? !version.memory_bits : 32)
  dll = getenv('bgkroot')+string(mbits,"('/lib/astron',i2,'.so')")
;
; Prepare arguments and then call the Fortran subroutine
  epoch_loc = double( epoch )
  ra_loc = float( ra )
  dec_loc = float( dec )
  iau_loc = bytarr( 3, n )
  result = call_external( dll, 'constellation_arg_', $
     n, epoch_loc, ra_loc, dec_loc, iau_loc )
;
; Return scalar or array, according to argument epoch
  if (size( epoch ))[0] eq 0 then $
     return, string(iau_loc[*,0]) $
  else $
     return, string(iau_loc)
  end