nova.sidereal_time source code

1 /*
2  *  This library is free software; you can redistribute it and/or
3  *  modify it under the terms of the GNU Lesser General Public
4  *  License as published by the Free Software Foundation; either
5  *  version 2 of the License, or (at your option) any later version.
6  *
7  *  This library is distributed in the hope that it will be useful,
8  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
9  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
10  *  Lesser General Public License for more details.
11  *
12  *  You should have received a copy of the GNU General Public License
13  *  along with this program; if not, write to the Free Software
14  *  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
15  *
16  *  Copyright (C) 2000 - 2005 Liam Girdwood <lgirdwood@gmail.com>
17  */
18 
19 module nova.sidereal_time;
20 
21 import std.math;
22 
23 import nova.nutation;
24 import nova.utility;
25 import nova.ln_types;
26 
27 extern (C) {
28 
29 /*! \fn double ln_get_mean_sidereal_time(double JD)
30 * \param JD Julian Day
31 * \return Mean sidereal time (hours).
32 *
33 * Calculate the mean sidereal time at the meridian of Greenwich of a given date.
34 */
35 /* Formula 11.1, 11.4 pg 83
36 */
37 
38 @nogc double ln_get_mean_sidereal_time(double JD) nothrow
39 {
40 	real sidereal;
41 	real T;
42 
43 	T =(JD - 2451545.0) / 36525.0;
44 
45 	/* calc mean angle */
46 	sidereal = 280.46061837 + (360.98564736629 *(JD - 2451545.0)) +
47 		(0.000387933 * T * T) - (T * T * T / 38710000.0);
48 
49 	/* add a convenient multiple of 360 degrees */
50 	sidereal = ln_range_degrees(sidereal);
51 
52 	/* change to hours */
53 	sidereal *= 24.0 / 360.0;
54 
55 	return sidereal;
56 }
57 
58 /*! \fn double ln_get_apparent_sidereal_time(double JD)
59 * \param JD Julian Day
60 * /return Apparent sidereal time (hours).
61 *
62 * Calculate the apparent sidereal time at the meridian of Greenwich of a given date,
63 * corrected for nutation.
64 */
65 /* Formula 11.1, 11.4 pg 83
66 */
67 
68 @nogc double ln_get_apparent_sidereal_time(double JD) nothrow
69 {
70 	double correction, sidereal;
71 	ln_nutation nutation;
72 
73 	/* get the mean sidereal time */
74 	sidereal = ln_get_mean_sidereal_time(JD);
75 
76 	/* add corrections for nutation in longitude and for the true obliquity of
77 	the ecliptic */
78 	ln_get_nutation(JD, nutation);
79 
80 	correction = (nutation.longitude / 15.0 *
81 	cos(ln_deg_to_rad(nutation.obliquity)));
82 
83 	sidereal += correction;
84 
85 	return sidereal;
86 }
87 
88 }