NAME:
  refrac
 PURPOSE:
  Apply atmospheric refraction to a ``true'' zenith angle
 DESCRIPTION:
  This calculation is based on a few different sources.  First, it is
  assumed that the index of refraction of air at the base of the atmosphere
  can be calculated (see AIRINDEX).  From the index of refraction, the
  bending is computed from the formula on p.55 of the old Explanatory Supplment
  to the Nautical Almanac.  This formula has been modified by removing the
  h/rho term.  The explanatory supplement doesn't indicate that this is
  legitimate but I've validated this computation against a more emperical
  formalism from Eisele and Shannon (NRL memo 3058, May 1975).  Eisele and
  Shannon don't indicate the wavelength of light used but if I use 0.56 microns
  and compare for the same input conditions (dry air only), the refraction
  computed agrees to within 1 arcsec down to 51 degrees zenith angle and
  is good to 10 arcsec down to 80 degrees.
 CATEGORY:
  Astronomy
 CALLING SEQUENCE:
  zref = refrac(z,wave,pressure,temp,relhum)
 INPUTS:
  z        - true zenith angle (as if there were no atmosphere), in radians
	wave     - wavelength of light, in microns
	pressure - atmospheric pressure in mm of Hg
	temp     - atmospheric temperature in degrees C
	relhum   - Relative humidity (in percent)

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

 OUTPUTS:
  returned is the refracted (or apparent) zenith distance.
 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:
  Not accurate (nor useful) for z > 85 degrees.

 PROCEDURE:

 MODIFICATION HISTORY:
  97/03/03, Written by Marc W. Buie, Lowell Observatory