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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 | |
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 | } |