mirror of https://github.com/XEphem/XEphem.git
53 lines
1.1 KiB
C
53 lines
1.1 KiB
C
/* compure parallactic angle: angle formed by N pole - Object - Zenith
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*/
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#include <stdio.h>
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#include <math.h>
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#include "astro.h"
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/* compute parallactic angle given latitude, object dec and alt.
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* all angles in rads.
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* N.B. always return >= 0, caller must determine sign and degenerate cases at
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* pole or zenith.
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*/
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double
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parallacticLDA (double lt, double dec, double alt)
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{
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double ca = sin(lt);
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double cb = sin(dec);
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double sb = cos(dec);
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double cc = sin(alt);
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double sc = cos(alt);
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double cpa;
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/* given three sides find an angle */
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if (sb==0 || sc==0)
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return (0);
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cpa = (ca - cb*cc)/(sb*sc);
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if (cpa < -1) cpa = -1;
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if (cpa > 1) cpa = 1;
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return (acos (cpa));
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}
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/* compute parallactic angle given latitude, object HA and Dec.
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* all angles in rads.
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* return value is between -PI and PI, sign is like HA, ie +west
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*/
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double
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parallacticLHD (double lt, double ha, double dec)
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{
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double A, b, cc, sc, B;
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A = ha;
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b = PI/2 - lt;
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cc = sin(dec);
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sc = cos(dec);
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solve_sphere (A, b, cc, sc, NULL, &B);
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if (B > PI)
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B -= 2*PI;
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return (B);
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}
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