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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: solar_declination.c 2369 2013-04-28 14:19:35Z swfltek $ */ | |
30 | ||
31 | /* | |
32 | Were it not for the eccentricity of Earths orbit, this would be a trivial function. | |
33 | ||
34 | We compute the Earths orbital position with respect to perihelion, from which we derive a | |
35 | 'velocity correction factor'. We then compute the orbital angle with respect to the | |
36 | December solstice, as 'modulated' by that correction factor. | |
37 | ||
38 | Due to the accumulation of rounding errors, the computed December solstice of 2135 will lag | |
39 | the actual solstice by many hours. A fudge factor, 'LAG', distributes the error across | |
40 | the 136 year range of this library. | |
41 | */ | |
42 | ||
43 | #include <time.h> | |
44 | #include <math.h> | |
45 | #include "ephemera_common.h" | |
46 | ||
47 | #define LAG 38520 | |
48 | ||
49 | double | |
50 | solar_declination(const time_t * timer) | |
51 | { | |
52 | ||
53 | uint32_t fT, oV; | |
54 | double dV, dT; | |
55 | ||
56 | /* Determine orbital angle relative to perihelion of January 1999 */ | |
57 | oV = *timer % ANOM_YEAR; | |
58 | oV += PERIHELION; | |
59 | dV = oV; | |
60 | dV /= ANOM_CYCLE; | |
61 | ||
62 | /* Derive velocity correction factor from the perihelion angle */ | |
63 | dV = sin(dV); | |
64 | dV *= DELTA_V; | |
65 | ||
66 | /* Determine orbital angle relative to solstice of December 1999 */ | |
67 | fT = *timer % TROP_YEAR; | |
68 | fT += SOLSTICE + LAG; | |
69 | dT = fT; | |
70 | dT /= TROP_CYCLE; | |
71 | dT += dV; | |
72 | ||
73 | /* Finally having the solstice angle, we can compute the declination */ | |
74 | dT = cos(dT) * INCLINATION; | |
75 | ||
76 | return -dT; | |
77 | } |