1 |
/*
|
2 |
|
3 |
SUNRISET.C - computes Sun rise/set times, start/end of twilight, and
|
4 |
the length of the day at any date and latitude
|
5 |
|
6 |
Written as DAYLEN.C, 1989-08-16
|
7 |
|
8 |
Modified to SUNRISET.C, 1992-12-01
|
9 |
|
10 |
(c) Paul Schlyter, 1989, 1992
|
11 |
|
12 |
Released to the public domain by Paul Schlyter, December 1992
|
13 |
|
14 |
*/
|
15 |
|
16 |
|
17 |
#include <stdio.h>
|
18 |
#include <math.h>
|
19 |
|
20 |
#include "sunriset.h"
|
21 |
|
22 |
/* The "workhorse" function for sun rise/set times */
|
23 |
|
24 |
int __sunriset__( int year, int month, int day, double lon, double lat,
|
25 |
double altit, int upper_limb, double *trise, double *tset )
|
26 |
/***************************************************************************/
|
27 |
/* Note: year,month,date = calendar date, 1801-2099 only. */
|
28 |
/* Eastern longitude positive, Western longitude negative */
|
29 |
/* Northern latitude positive, Southern latitude negative */
|
30 |
/* The longitude value IS critical in this function! */
|
31 |
/* altit = the altitude which the Sun should cross */
|
32 |
/* Set to -35/60 degrees for rise/set, -6 degrees */
|
33 |
/* for civil, -12 degrees for nautical and -18 */
|
34 |
/* degrees for astronomical twilight. */
|
35 |
/* upper_limb: non-zero -> upper limb, zero -> center */
|
36 |
/* Set to non-zero (e.g. 1) when computing rise/set */
|
37 |
/* times, and to zero when computing start/end of */
|
38 |
/* twilight. */
|
39 |
/* *rise = where to store the rise time */
|
40 |
/* *set = where to store the set time */
|
41 |
/* Both times are relative to the specified altitude, */
|
42 |
/* and thus this function can be used to comupte */
|
43 |
/* various twilight times, as well as rise/set times */
|
44 |
/* Return value: 0 = sun rises/sets this day, times stored at */
|
45 |
/* *trise and *tset. */
|
46 |
/* +1 = sun above the specified "horizon" 24 hours. */
|
47 |
/* *trise set to time when the sun is at south, */
|
48 |
/* minus 12 hours while *tset is set to the south */
|
49 |
/* time plus 12 hours. "Day" length = 24 hours */
|
50 |
/* -1 = sun is below the specified "horizon" 24 hours */
|
51 |
/* "Day" length = 0 hours, *trise and *tset are */
|
52 |
/* both set to the time when the sun is at south. */
|
53 |
/* */
|
54 |
/**********************************************************************/
|
55 |
{
|
56 |
double d, /* Days since 2000 Jan 0.0 (negative before) */
|
57 |
sr, /* Solar distance, astronomical units */
|
58 |
sRA, /* Sun's Right Ascension */
|
59 |
sdec, /* Sun's declination */
|
60 |
sradius, /* Sun's apparent radius */
|
61 |
t, /* Diurnal arc */
|
62 |
tsouth, /* Time when Sun is at south */
|
63 |
sidtime; /* Local sidereal time */
|
64 |
|
65 |
int rc = 0; /* Return cde from function - usually 0 */
|
66 |
|
67 |
/* Compute d of 12h local mean solar time */
|
68 |
d = days_since_2000_Jan_0(year,month,day) + 0.5 - lon/360.0;
|
69 |
|
70 |
/* Compute local sideral time of this moment */
|
71 |
sidtime = revolution( GMST0(d) + 180.0 + lon );
|
72 |
|
73 |
/* Compute Sun's RA + Decl at this moment */
|
74 |
sun_RA_dec( d, &sRA, &sdec, &sr );
|
75 |
|
76 |
/* Compute time when Sun is at south - in hours UT */
|
77 |
tsouth = 12.0 - rev180(sidtime - sRA)/15.0;
|
78 |
|
79 |
/* Compute the Sun's apparent radius, degrees */
|
80 |
sradius = 0.2666 / sr;
|
81 |
|
82 |
/* Do correction to upper limb, if necessary */
|
83 |
if ( upper_limb )
|
84 |
altit -= sradius;
|
85 |
|
86 |
/* Compute the diurnal arc that the Sun traverses to reach */
|
87 |
/* the specified altitide altit: */
|
88 |
{
|
89 |
double cost;
|
90 |
cost = ( sind(altit) - sind(lat) * sind(sdec) ) /
|
91 |
( cosd(lat) * cosd(sdec) );
|
92 |
if ( cost >= 1.0 )
|
93 |
rc = -1, t = 0.0; /* Sun always below altit */
|
94 |
else if ( cost <= -1.0 )
|
95 |
rc = +1, t = 12.0; /* Sun always above altit */
|
96 |
else
|
97 |
t = acosd(cost)/15.0; /* The diurnal arc, hours */
|
98 |
}
|
99 |
|
100 |
/* Store rise and set times - in hours UT */
|
101 |
*trise = tsouth - t;
|
102 |
*tset = tsouth + t;
|
103 |
|
104 |
return rc;
|
105 |
} /* __sunriset__ */
|
106 |
|
107 |
|
108 |
|
109 |
/* The "workhorse" function */
|
110 |
|
111 |
|
112 |
double __daylen__( int year, int month, int day, double lon, double lat,
|
113 |
double altit, int upper_limb )
|
114 |
/**********************************************************************/
|
115 |
/* Note: year,month,date = calendar date, 1801-2099 only. */
|
116 |
/* Eastern longitude positive, Western longitude negative */
|
117 |
/* Northern latitude positive, Southern latitude negative */
|
118 |
/* The longitude value is not critical. Set it to the correct */
|
119 |
/* longitude if you're picky, otherwise set to to, say, 0.0 */
|
120 |
/* The latitude however IS critical - be sure to get it correct */
|
121 |
/* altit = the altitude which the Sun should cross */
|
122 |
/* Set to -35/60 degrees for rise/set, -6 degrees */
|
123 |
/* for civil, -12 degrees for nautical and -18 */
|
124 |
/* degrees for astronomical twilight. */
|
125 |
/* upper_limb: non-zero -> upper limb, zero -> center */
|
126 |
/* Set to non-zero (e.g. 1) when computing day length */
|
127 |
/* and to zero when computing day+twilight length. */
|
128 |
/**********************************************************************/
|
129 |
{
|
130 |
double d, /* Days since 2000 Jan 0.0 (negative before) */
|
131 |
obl_ecl, /* Obliquity (inclination) of Earth's axis */
|
132 |
sr, /* Solar distance, astronomical units */
|
133 |
slon, /* True solar longitude */
|
134 |
sin_sdecl, /* Sine of Sun's declination */
|
135 |
cos_sdecl, /* Cosine of Sun's declination */
|
136 |
sradius, /* Sun's apparent radius */
|
137 |
t; /* Diurnal arc */
|
138 |
|
139 |
/* Compute d of 12h local mean solar time */
|
140 |
d = days_since_2000_Jan_0(year,month,day) + 0.5 - lon/360.0;
|
141 |
|
142 |
/* Compute obliquity of ecliptic (inclination of Earth's axis) */
|
143 |
obl_ecl = 23.4393 - 3.563E-7 * d;
|
144 |
|
145 |
/* Compute Sun's position */
|
146 |
sunpos( d, &slon, &sr );
|
147 |
|
148 |
/* Compute sine and cosine of Sun's declination */
|
149 |
sin_sdecl = sind(obl_ecl) * sind(slon);
|
150 |
cos_sdecl = sqrt( 1.0 - sin_sdecl * sin_sdecl );
|
151 |
|
152 |
/* Compute the Sun's apparent radius, degrees */
|
153 |
sradius = 0.2666 / sr;
|
154 |
|
155 |
/* Do correction to upper limb, if necessary */
|
156 |
if ( upper_limb )
|
157 |
altit -= sradius;
|
158 |
|
159 |
/* Compute the diurnal arc that the Sun traverses to reach */
|
160 |
/* the specified altitide altit: */
|
161 |
{
|
162 |
double cost;
|
163 |
cost = ( sind(altit) - sind(lat) * sin_sdecl ) /
|
164 |
( cosd(lat) * cos_sdecl );
|
165 |
if ( cost >= 1.0 )
|
166 |
t = 0.0; /* Sun always below altit */
|
167 |
else if ( cost <= -1.0 )
|
168 |
t = 24.0; /* Sun always above altit */
|
169 |
else t = (2.0/15.0) * acosd(cost); /* The diurnal arc, hours */
|
170 |
}
|
171 |
return t;
|
172 |
} /* __daylen__ */
|
173 |
|
174 |
|
175 |
/* This function computes the Sun's position at any instant */
|
176 |
|
177 |
void sunpos( double d, double *lon, double *r )
|
178 |
/******************************************************/
|
179 |
/* Computes the Sun's ecliptic longitude and distance */
|
180 |
/* at an instant given in d, number of days since */
|
181 |
/* 2000 Jan 0.0. The Sun's ecliptic latitude is not */
|
182 |
/* computed, since it's always very near 0. */
|
183 |
/******************************************************/
|
184 |
{
|
185 |
double M, /* Mean anomaly of the Sun */
|
186 |
w, /* Mean longitude of perihelion */
|
187 |
/* Note: Sun's mean longitude = M + w */
|
188 |
e, /* Eccentricity of Earth's orbit */
|
189 |
E, /* Eccentric anomaly */
|
190 |
x, y, /* x, y coordinates in orbit */
|
191 |
v; /* True anomaly */
|
192 |
|
193 |
/* Compute mean elements */
|
194 |
M = revolution( 356.0470 + 0.9856002585 * d );
|
195 |
w = 282.9404 + 4.70935E-5 * d;
|
196 |
e = 0.016709 - 1.151E-9 * d;
|
197 |
|
198 |
/* Compute true longitude and radius vector */
|
199 |
E = M + e * RADEG * sind(M) * ( 1.0 + e * cosd(M) );
|
200 |
x = cosd(E) - e;
|
201 |
y = sqrt( 1.0 - e*e ) * sind(E);
|
202 |
*r = sqrt( x*x + y*y ); /* Solar distance */
|
203 |
v = atan2d( y, x ); /* True anomaly */
|
204 |
*lon = v + w; /* True solar longitude */
|
205 |
if ( *lon >= 360.0 )
|
206 |
*lon -= 360.0; /* Make it 0..360 degrees */
|
207 |
}
|
208 |
|
209 |
void sun_RA_dec( double d, double *RA, double *dec, double *r )
|
210 |
{
|
211 |
double lon, obl_ecl;
|
212 |
double xs, ys, zs;
|
213 |
double xe, ye, ze;
|
214 |
|
215 |
/* Compute Sun's ecliptical coordinates */
|
216 |
sunpos( d, &lon, r );
|
217 |
|
218 |
/* Compute ecliptic rectangular coordinates */
|
219 |
xs = *r * cosd(lon);
|
220 |
ys = *r * sind(lon);
|
221 |
zs = 0; /* because the Sun is always in the ecliptic plane! */
|
222 |
|
223 |
/* Compute obliquity of ecliptic (inclination of Earth's axis) */
|
224 |
obl_ecl = 23.4393 - 3.563E-7 * d;
|
225 |
|
226 |
/* Convert to equatorial rectangular coordinates - x is unchanged */
|
227 |
xe = xs;
|
228 |
ye = ys * cosd(obl_ecl);
|
229 |
ze = ys * sind(obl_ecl);
|
230 |
|
231 |
/* Convert to spherical coordinates */
|
232 |
*RA = atan2d( ye, xe );
|
233 |
*dec = atan2d( ze, sqrt(xe*xe + ye*ye) );
|
234 |
|
235 |
} /* sun_RA_dec */
|
236 |
|
237 |
|
238 |
/******************************************************************/
|
239 |
/* This function reduces any angle to within the first revolution */
|
240 |
/* by subtracting or adding even multiples of 360.0 until the */
|
241 |
/* result is >= 0.0 and < 360.0 */
|
242 |
/******************************************************************/
|
243 |
|
244 |
#define INV360 ( 1.0 / 360.0 )
|
245 |
|
246 |
double revolution( double x )
|
247 |
/*****************************************/
|
248 |
/* Reduce angle to within 0..360 degrees */
|
249 |
/*****************************************/
|
250 |
{
|
251 |
return( x - 360.0 * floor( x * INV360 ) );
|
252 |
} /* revolution */
|
253 |
|
254 |
double rev180( double x )
|
255 |
/*********************************************/
|
256 |
/* Reduce angle to within -180..+180 degrees */
|
257 |
/*********************************************/
|
258 |
{
|
259 |
return( x - 360.0 * floor( x * INV360 + 0.5 ) );
|
260 |
} /* revolution */
|
261 |
|
262 |
|
263 |
/*******************************************************************/
|
264 |
/* This function computes GMST0, the Greenwhich Mean Sidereal Time */
|
265 |
/* at 0h UT (i.e. the sidereal time at the Greenwhich meridian at */
|
266 |
/* 0h UT). GMST is then the sidereal time at Greenwich at any */
|
267 |
/* time of the day. I've generelized GMST0 as well, and define it */
|
268 |
/* as: GMST0 = GMST - UT -- this allows GMST0 to be computed at */
|
269 |
/* other times than 0h UT as well. While this sounds somewhat */
|
270 |
/* contradictory, it is very practical: instead of computing */
|
271 |
/* GMST like: */
|
272 |
/* */
|
273 |
/* GMST = (GMST0) + UT * (366.2422/365.2422) */
|
274 |
/* */
|
275 |
/* where (GMST0) is the GMST last time UT was 0 hours, one simply */
|
276 |
/* computes: */
|
277 |
/* */
|
278 |
/* GMST = GMST0 + UT */
|
279 |
/* */
|
280 |
/* where GMST0 is the GMST "at 0h UT" but at the current moment! */
|
281 |
/* Defined in this way, GMST0 will increase with about 4 min a */
|
282 |
/* day. It also happens that GMST0 (in degrees, 1 hr = 15 degr) */
|
283 |
/* is equal to the Sun's mean longitude plus/minus 180 degrees! */
|
284 |
/* (if we neglect aberration, which amounts to 20 seconds of arc */
|
285 |
/* or 1.33 seconds of time) */
|
286 |
/* */
|
287 |
/*******************************************************************/
|
288 |
|
289 |
double GMST0( double d )
|
290 |
{
|
291 |
double sidtim0;
|
292 |
/* Sidtime at 0h UT = L (Sun's mean longitude) + 180.0 degr */
|
293 |
/* L = M + w, as defined in sunpos(). Since I'm too lazy to */
|
294 |
/* add these numbers, I'll let the C compiler do it for me. */
|
295 |
/* Any decent C compiler will add the constants at compile */
|
296 |
/* time, imposing no runtime or code overhead. */
|
297 |
sidtim0 = revolution( ( 180.0 + 356.0470 + 282.9404 ) +
|
298 |
( 0.9856002585 + 4.70935E-5 ) * d );
|
299 |
return sidtim0;
|
300 |
} /* GMST0 */
|