1 /*
2 * (C)2012 Michael Duane Rice All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions are
6 * met:
7 *
8 * Redistributions of source code must retain the above copyright notice, this
9 * list of conditions and the following disclaimer. Redistributions in binary
10 * form must reproduce the above copyright notice, this list of conditions
11 * and the following disclaimer in the documentation and/or other materials
12 * provided with the distribution. Neither the name of the copyright holders
13 * nor the names of contributors may be used to endorse or promote products
14 * derived from this software without specific prior written permission.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
17 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
20 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
21 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
22 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
23 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
24 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
25 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
26 * POSSIBILITY OF SUCH DAMAGE.
27 */
28
29 /* $Id: daylight_seconds.c 2369 2013-04-28 14:19:35Z swfltek $ */
30
31 /*
32 Determine the amount of time the sun is above the horizon. At high latitudes, around the
33 solstices, this can be zero or greater than ONE_DAY.
34
35 */
36
37 #include <time.h>
38 #include <math.h>
39
40 extern long __latitude;
41
42 long
daylight_seconds(const time_t * timer)43 daylight_seconds(const time_t * timer)
44 {
45 double l, d;
46 long n;
47
48 /* convert latitude to radians */
49 l = __latitude / 206264.806;
50
51 d = -solar_declination(timer);
52
53 /* partial 'Sunrise Equation' */
54 d = tan(l) * tan(d);
55
56 /* magnitude of d may exceed 1.0 at near solstices */
57 if (d > 1.0)
58 d = 1.0;
59
60 if (d < -1.0)
61 d = -1.0;
62
63 /* derive hour angle */
64 d = acos(d);
65
66 /* but for atmospheric refraction, this would be d /= M_PI */
67 d /= 3.112505;
68
69 n = ONE_DAY * d;
70
71 return n;
72 }
73