; ----------------------------------------------------------------------
;  Compute apparent zenith angle at 4000 Angstroms, given the
;  true zenith angle in degrees.  Return -99. if the true angle is  
;  too large (>80) to do an accurate calculation.  Assume pressure=760 mm Hg
;  and T=15 deg Celius, dry air, from CRC and other sources
;  Returns apparent zenith angle in degrees.            -tab 10oct2012
;
function gal_apparent_zenith_angle, tzd

  if ((tzd lt 0.)||(tzd gt 80.)) then return,-99.
  deg = !dpi / 180.d            ;>
  azd=deg*(tzd)
  i=0
  differr=1.0                   ;
  while ((i lt 40)&&(differr gt 1.e-10)) do begin
     i = i + 1
     oldazd = azd
     r = tan(azd)
     r = (58.16 * r) - (0.067 * r * r * r)
     azd = deg*(tzd) - deg*(r / 3600.)
     differr = ABS(( oldazd - azd ))
  endwhile
  if (i gt 39) then begin
     print,"===warning: gal_apparent_zenith_angle: too many iterations."
     return,-99.
  endif
  return, (azd)/deg

end