| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | void slaH2e ( float az, float el, float phi, float *ha, float *dec )
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| 4 | /*
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| 5 | ** - - - - - - -
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| 6 | ** s l a H 2 e
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| 7 | ** - - - - - - -
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| 8 | **
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| 9 | ** Horizon to equatorial coordinates: Az,El to HA,Dec
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| 10 | **
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| 11 | ** (single precision)
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| 12 | **
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| 13 | ** Given:
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| 14 | ** az float azimuth
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| 15 | ** el float elevation
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| 16 | ** phi float observatory latitude
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| 17 | **
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| 18 | ** Returned:
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| 19 | ** *ha float hour angle
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| 20 | ** *dec float declination
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| 21 | **
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| 22 | ** Notes:
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| 23 | **
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| 24 | ** 1) All the arguments are angles in radians.
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| 25 | **
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| 26 | ** 2) The sign convention for azimuth is north zero, east +pi/2.
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| 27 | **
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| 28 | ** 3) HA is returned in the range +/-pi. Declination is returned
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| 29 | ** in the range +/-pi/2.
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| 30 | **
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| 31 | ** 4) The latitude is (in principle) geodetic. In critical
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| 32 | ** applications, corrections for polar motion should be applied.
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| 33 | **
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| 34 | ** 5) In some applications it will be important to specify the
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| 35 | ** correct type of elevation in order to produce the required
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| 36 | ** type of HA,Dec. In particular, it may be important to
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| 37 | ** distinguish between the elevation as affected by refraction,
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| 38 | ** which will yield the "observed" HA,Dec, and the elevation
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| 39 | ** in vacuo, which will yield the "topocentric" HA,Dec. If the
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| 40 | ** effects of diurnal aberration can be neglected, the
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| 41 | ** topocentric HA,Dec may be used as an approximation to the
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| 42 | ** "apparent" HA,Dec.
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| 43 | **
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| 44 | ** 6) No range checking of arguments is done.
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| 45 | **
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| 46 | ** 7) In applications which involve many such calculations, rather
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| 47 | ** than calling the present routine it will be more efficient to
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| 48 | ** use inline code, having previously computed fixed terms such
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| 49 | ** as sine and cosine of latitude.
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| 50 | **
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| 51 | ** Last revision: 21 February 1996
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| 52 | **
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| 53 | ** Copyright P.T.Wallace. All rights reserved.
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| 54 | */
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| 55 | {
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| 56 | float sa, ca, se, ce, sp, cp, x, y, z, r;
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| 57 |
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| 58 | /* Useful trig functions */
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| 59 | sa = (float) sin ( az );
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| 60 | ca = (float) cos ( az );
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| 61 | se = (float) sin ( el );
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| 62 | ce = (float) cos ( el );
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| 63 | sp = (float) sin ( phi );
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| 64 | cp = (float) cos ( phi );
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| 65 |
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| 66 | /* HA,Dec as x,y,z */
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| 67 | x = - ca * ce * sp + se * cp;
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| 68 | y = - sa * ce;
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| 69 | z = ca * ce * cp + se * sp;
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| 70 |
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| 71 | /* To spherical */
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| 72 | r = (float) sqrt ( x * x + y * y );
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| 73 | *ha = ( r == 0.0f ) ? 0.0f : (float) atan2 ( y, x ) ;
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| 74 | *dec = (float) atan2 ( z, r );
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| 75 | }
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