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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: moon_phase.c 2365 2013-04-27 15:32:59Z swfltek $ */ | |
30 | ||
31 | /* | |
32 | Return an approximation to the phase of the moon. Since no attempt is made to account for | |
33 | Sol, Jupiter or Venus, it will often be off by several hours. | |
34 | */ | |
35 | ||
36 | #include <time.h> | |
37 | #include <math.h> | |
38 | ||
39 | int8_t | |
40 | moon_phase(const time_t * timestamp) | |
41 | { | |
42 | uint32_t t; | |
43 | int32_t n; | |
44 | double mc; | |
45 | ||
46 | /* refer to first new moon of the epoch */ | |
47 | t = *timestamp - 1744800UL; | |
48 | ||
49 | /* constrain to 1 lunar cycle */ | |
50 | n = t % 2551443UL; | |
51 | ||
52 | /* offset by 1/2 lunar cycle */ | |
53 | n -= 1275721L; | |
54 | mc = n; | |
55 | mc /= 1275721.0; | |
56 | mc *= M_PI; | |
57 | mc = cos(mc) * sin(mc); | |
58 | mc *= 12.5; | |
59 | ||
60 | /* scale to range - 100...+ 100 */ | |
61 | n /= 12757L; | |
62 | n -= mc; | |
63 | ||
64 | return n; | |
65 | } |