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DOUBLE PRECISION FUNCTION sla_AIRMAS (ZD)
*+
* - - - - - - -
* A I R M A 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
*-
IMPLICIT NONE
DOUBLE PRECISION ZD
DOUBLE PRECISION SECZM1
SECZM1 = 1D0/(COS(MIN(1.52D0,ABS(ZD))))-1D0
sla_AIRMAS = 1D0 + SECZM1*(0.9981833D0
: - SECZM1*(0.002875D0 + 0.0008083D0*SECZM1))
END
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