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.help airmas Jun99 "Slalib Package"
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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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