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      DOUBLE PRECISION FUNCTION slARMS (ZD)

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      A R M S
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  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.

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