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