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diff --git a/noao/obsutil/src/pairmass/airmass.x b/noao/obsutil/src/pairmass/airmass.x
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+++ b/noao/obsutil/src/pairmass/airmass.x
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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