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include <math.h>
# AIRMASS -- Compute airmass from DEC, LATITUDE and HA
# Airmass formulation from Allen "Astrophysical Quantities" 1973 p.125,133.
# and John Ball's book on Algorithms for the HP-45
double procedure airmass (ha, dec, lat)
double ha, dec, lat, cos_zd, x
define SCALE 750.0d0 # Atmospheric scale height
begin
if (IS_INDEFD (ha) || IS_INDEFD (dec) || IS_INDEFD (lat))
call error (1, "Can't determine airmass")
cos_zd = sin(DEGTORAD(lat)) * sin(DEGTORAD(dec)) +
cos(DEGTORAD(lat)) * cos(DEGTORAD(dec)) * cos(DEGTORAD(ha*15.))
x = SCALE * cos_zd
return (sqrt (x**2 + 2*SCALE + 1) - x)
end
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