source: trunk/MagicSoft/slalib/pa.c@ 11169

Last change on this file since 11169 was 731, checked in by tbretz, 24 years ago
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1#include "slalib.h"
2#include "slamac.h"
3double slaPa ( double ha, double dec, double phi )
4/*+
5** - - - - - -
6** s l a P a
7** - - - - - -
8**
9** HA, Dec to Parallactic Angle.
10**
11** (double precision)
12**
13** Given:
14** ha d hour angle in radians (geocentric apparent)
15** dec d declination in radians (geocentric apparent)
16** phi d observatory latitude in radians (geodetic)
17**
18** The result is in the range -pi to +pi
19**
20** Notes:
21**
22** 1) The parallactic angle at a point in the sky is the position
23** angle of the vertical, i.e. the angle between the direction to
24** the pole and to the zenith. In precise applications care must
25** be taken only to use geocentric apparent HA,Dec and to consider
26** separately the effects of atmospheric refraction and telescope
27** mount errors.
28**
29** 2) At the pole a zero result is returned.
30**
31** Last revision: 16 August 1994
32**
33** Copyright P.T.Wallace. All rights reserved.
34*/
35{
36 double cp, cqsz, sqsz;
37
38 cp = cos ( phi );
39 sqsz = cp * sin ( ha );
40 cqsz = sin ( phi ) * cos ( dec) - cp * sin ( dec) * cos ( ha );
41 return ( ( sqsz != 0.0 || cqsz != 0.0 ) ? atan2 ( sqsz, cqsz ) : 0.0 );
42}
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