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