mirror of https://github.com/XEphem/XEphem.git
25 lines
595 B
C
25 lines
595 B
C
#include <math.h>
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#include "astro.h"
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/* given apparent altitude find airmass.
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* R.H. Hardie, 1962, `Photoelectric Reductions', Chapter 8 of Astronomical
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* Techniques, W.A. Hiltner (Ed), Stars and Stellar Systems, II (University
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* of Chicago Press: Chicago), pp178-208.
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*/
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void
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airmass (
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double aa, /* apparent altitude, rads */
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double *Xp) /* airmasses */
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{
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double sm1; /* secant zenith angle, minus 1 */
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/* degenerate near or below horizon */
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if (aa < degrad(3.0))
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aa = degrad(3.0);
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sm1 = 1.0/sin(aa) - 1.0;
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*Xp = 1.0 + sm1*(0.9981833 - sm1*(0.002875 + 0.0008083*sm1));
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}
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