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+.help airmas Jun99 "Slalib Package"
+.nf
+
+      DOUBLE PRECISION FUNCTION slARMS (ZD)
+
+     - - - - - - -
+      A R M S
+     - - - - - - -
+
+  Air mass at given zenith distance (double precision)
+
+  Given:
+     ZD     d     Observed zenith distance (radians)
+
+  The result is an estimate of the air mass, in units of that
+  at the zenith.
+
+  Notes:
+
+  1)  The "observed" zenith distance referred to above means "as
+      affected by refraction".
+
+  2)  Uses Hardie's (1962) polynomial fit to Bemporad's data for
+      the relative air mass, X, in units of thickness at the zenith
+      as tabulated by Schoenberg (1929). This is adequate for all
+      normal needs as it is accurate to better than 0.1% up to X =
+      6.8 and better than 1% up to X = 10. Bemporad's tabulated
+      values are unlikely to be trustworthy to such accuracy
+      because of variations in density, pressure and other
+      conditions in the atmosphere from those assumed in his work.
+
+  3)  The sign of the ZD is ignored.
+
+  4)  At zenith distances greater than about ZD = 87 degrees the
+      air mass is held constant to avoid arithmetic overflows.
+
+  References:
+     Hardie, R.H., 1962, in "Astronomical Techniques"
+        ed. W.A. Hiltner, University of Chicago Press, p180.
+     Schoenberg, E., 1929, Hdb. d. Ap.,
+        Berlin, Julius Springer, 2, 268.
+
+  Original code by P.W.Hill, St Andrews
+
+  P.T.Wallace   Starlink   18 March 1999
+
+  Copyright (C) 1999 Rutherford Appleton Laboratory
+  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+
+.fi
+.endhelp