| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | double slaZd ( double ha, double dec, double phi )
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| 4 | /*
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| 5 | ** - - - - - -
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| 6 | ** s l a Z d
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| 7 | ** - - - - - -
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| 8 | **
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| 9 | ** HA, Dec to Zenith Distance.
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| 10 | **
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| 11 | ** (double precision)
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| 12 | **
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| 13 | ** Given:
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| 14 | ** ha double Hour Angle in radians
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| 15 | ** dec double declination in radians
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| 16 | ** phi double observatory latitude in radians
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| 17 | **
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| 18 | ** The result is in the range 0 to pi.
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| 19 | **
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| 20 | ** Notes:
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| 21 | **
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| 22 | ** 1) The latitude must be geodetic. In critical applications,
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| 23 | ** corrections for polar motion should be applied.
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| 24 | **
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| 25 | ** 2) In some applications it will be important to specify the
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| 26 | ** correct type of hour angle and declination in order to
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| 27 | ** produce the required type of zenith distance. In particular,
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| 28 | ** it may be important to distinguish between the zenith distance
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| 29 | ** as affected by refraction, which would require the "observed"
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| 30 | ** HA,Dec, and the zenith distance in vacuo, which would require
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| 31 | ** the "topocentric" HA,Dec. If the effects of diurnal aberration
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| 32 | ** can be neglected, the "apparent" HA,Dec may be used instead of
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| 33 | ** the topocentric HA,Dec.
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| 34 | **
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| 35 | ** 3) No range checking of arguments is done.
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| 36 | **
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| 37 | ** 4) In applications which involve many zenith distance calculations,
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| 38 | ** rather than calling the present routine it will be more efficient
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| 39 | ** to use inline code, having previously computed fixed terms such
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| 40 | ** as sine and cosine of latitude, and perhaps sine and cosine of
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| 41 | ** declination.
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| 42 | **
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| 43 | ** Last revision: 4 April 1994
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| 44 | **
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| 45 | ** Copyright P.T.Wallace. All rights reserved.
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| 46 | */
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| 47 | {
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| 48 | double sh, ch, sd, cd, sp, cp, x, y, z;
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| 49 |
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| 50 | sh = sin ( ha );
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| 51 | ch = cos ( ha );
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| 52 | sd = sin ( dec );
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| 53 | cd = cos ( dec );
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| 54 | sp = sin ( phi );
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| 55 | cp = cos ( phi );
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| 56 |
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| 57 | x = ch * cd * sp - sd * cp;
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| 58 | y = sh * cd;
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| 59 | z = ch * cd * cp + sd * sp;
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| 60 |
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| 61 | return atan2 ( sqrt ( x * x + y * y ), z );
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| 62 | }
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