summaryrefslogtreecommitdiff
path: root/ext/date/lib/astro.c
blob: bf6b89b4173791d1d57a753505a94cfb6cbf4646 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
/*
   +----------------------------------------------------------------------+
   | PHP Version 5                                                        |
   +----------------------------------------------------------------------+
   | Copyright (c) 1997-2006 The PHP Group                                |
   +----------------------------------------------------------------------+
   | This source file is subject to version 3.01 of the PHP license,      |
   | that is bundled with this package in the file LICENSE, and is        |
   | available through the world-wide-web at the following url:           |
   | http://www.php.net/license/3_01.txt                                  |
   | If you did not receive a copy of the PHP license and are unable to   |
   | obtain it through the world-wide-web, please send a note to          |
   | license@php.net so we can mail you a copy immediately.               |
   +----------------------------------------------------------------------+
   | Algorithms are taken from a public domain source by Paul             |
   | Schlyter, who wrote this in December 1992                            |
   +----------------------------------------------------------------------+
   | Authors: Derick Rethans <derick@derickrethans.nl>                    |
   +----------------------------------------------------------------------+
 */

/* $Id$ */

#include <stdio.h>
#include <math.h>
#include "timelib.h"

#define days_since_2000_Jan_0(y,m,d) \
	(367L*(y)-((7*((y)+(((m)+9)/12)))/4)+((275*(m))/9)+(d)-730530L)

#ifndef PI
 #define PI        3.1415926535897932384
#endif

#define RADEG     ( 180.0 / PI )
#define DEGRAD    ( PI / 180.0 )

/* The trigonometric functions in degrees */

#define sind(x)  sin((x)*DEGRAD)
#define cosd(x)  cos((x)*DEGRAD)
#define tand(x)  tan((x)*DEGRAD)

#define atand(x)    (RADEG*atan(x))
#define asind(x)    (RADEG*asin(x))
#define acosd(x)    (RADEG*acos(x))
#define atan2d(y,x) (RADEG*atan2(y,x))


/* Following are some macros around the "workhorse" function __daylen__ */
/* They mainly fill in the desired values for the reference altitude    */
/* below the horizon, and also selects whether this altitude should     */
/* refer to the Sun's center or its upper limb.                         */


#include "astro.h"

/******************************************************************/
/* This function reduces any angle to within the first revolution */
/* by subtracting or adding even multiples of 360.0 until the     */
/* result is >= 0.0 and < 360.0                                   */
/******************************************************************/

#define INV360    (1.0 / 360.0)

/*****************************************/
/* Reduce angle to within 0..360 degrees */
/*****************************************/
static double astro_revolution(double x)
{
	return (x - 360.0 * floor(x * INV360));
}

/*********************************************/
/* Reduce angle to within +180..+180 degrees */
/*********************************************/
static double astro_rev180( double x )
{
	return (x - 360.0 * floor(x * INV360 + 0.5));
}

/*******************************************************************/
/* This function computes GMST0, the Greenwich Mean Sidereal Time  */
/* at 0h UT (i.e. the sidereal time at the Greenwhich meridian at  */
/* 0h UT).  GMST is then the sidereal time at Greenwich at any     */
/* time of the day.  I've generalized GMST0 as well, and define it */
/* as:  GMST0 = GMST - UT  --  this allows GMST0 to be computed at */
/* other times than 0h UT as well.  While this sounds somewhat     */
/* contradictory, it is very practical:  instead of computing      */
/* GMST like:                                                      */
/*                                                                 */
/*  GMST = (GMST0) + UT * (366.2422/365.2422)                      */
/*                                                                 */
/* where (GMST0) is the GMST last time UT was 0 hours, one simply  */
/* computes:                                                       */
/*                                                                 */
/*  GMST = GMST0 + UT                                              */
/*                                                                 */
/* where GMST0 is the GMST "at 0h UT" but at the current moment!   */
/* Defined in this way, GMST0 will increase with about 4 min a     */
/* day.  It also happens that GMST0 (in degrees, 1 hr = 15 degr)   */
/* is equal to the Sun's mean longitude plus/minus 180 degrees!    */
/* (if we neglect aberration, which amounts to 20 seconds of arc   */
/* or 1.33 seconds of time)                                        */
/*                                                                 */
/*******************************************************************/

static double astro_GMST0(double d)
{
	double sidtim0;
	/* Sidtime at 0h UT = L (Sun's mean longitude) + 180.0 degr  */
	/* L = M + w, as defined in sunpos().  Since I'm too lazy to */
	/* add these numbers, I'll let the C compiler do it for me.  */
	/* Any decent C compiler will add the constants at compile   */
	/* time, imposing no runtime or code overhead.               */
	sidtim0 = astro_revolution((180.0 + 356.0470 + 282.9404) + (0.9856002585 + 4.70935E-5) * d);
	return sidtim0;
} 

/* This function computes the Sun's position at any instant */

/******************************************************/
/* Computes the Sun's ecliptic longitude and distance */
/* at an instant given in d, number of days since     */
/* 2000 Jan 0.0.  The Sun's ecliptic latitude is not  */
/* computed, since it's always very near 0.           */
/******************************************************/
static void astro_sunpos(double d, double *lon, double *r)
{
	double M,         /* Mean anomaly of the Sun */
	       w,         /* Mean longitude of perihelion */
	                  /* Note: Sun's mean longitude = M + w */
	       e,         /* Eccentricity of Earth's orbit */
	       E,         /* Eccentric anomaly */
	       x, y,      /* x, y coordinates in orbit */
	       v;         /* True anomaly */

	/* Compute mean elements */
	M = astro_revolution(356.0470 + 0.9856002585 * d);
	w = 282.9404 + 4.70935E-5 * d;
	e = 0.016709 - 1.151E-9 * d;

	/* Compute true longitude and radius vector */
	E = M + e * RADEG * sind(M) * (1.0 + e * cosd(M));
	x = cosd(E) - e;
	y = sqrt(1.0 - e*e) * sind(E);
	*r = sqrt(x*x + y*y);              /* Solar distance */
	v = atan2d(y, x);                  /* True anomaly */
	*lon = v + w;                        /* True solar longitude */
	if (*lon >= 360.0) {
		*lon -= 360.0;                   /* Make it 0..360 degrees */
	}
}

static void astro_sun_RA_dec(double d, double *RA, double *dec, double *r)
{
	double lon, obl_ecl, x, y, z;

	/* Compute Sun's ecliptical coordinates */
	astro_sunpos(d, &lon, r);

	/* Compute ecliptic rectangular coordinates (z=0) */
	x = *r * cosd(lon);
	y = *r * sind(lon);

	/* Compute obliquity of ecliptic (inclination of Earth's axis) */
	obl_ecl = 23.4393 - 3.563E-7 * d;

	/* Convert to equatorial rectangular coordinates - x is unchanged */
	z = y * sind(obl_ecl);
	y = y * cosd(obl_ecl);

	/* Convert to spherical coordinates */
	*RA = atan2d(y, x);
	*dec = atan2d(z, sqrt(x*x + y*y));
}

/**
 * Note: timestamp = unixtimestamp (NEEDS to be 00:00:00 UT)
 *       Eastern longitude positive, Western longitude negative       
 *       Northern latitude positive, Southern latitude negative       
 *       The longitude value IS critical in this function!            
 *       altit = the altitude which the Sun should cross              
 *               Set to -35/60 degrees for rise/set, -6 degrees       
 *               for civil, -12 degrees for nautical and -18          
 *               degrees for astronomical twilight.                   
 *         upper_limb: non-zero -> upper limb, zero -> center         
 *               Set to non-zero (e.g. 1) when computing rise/set     
 *               times, and to zero when computing start/end of       
 *               twilight.                                            
 *        *rise = where to store the rise time                        
 *        *set  = where to store the set  time                        
 *                Both times are relative to the specified altitude,  
 *                and thus this function can be used to compute       
 *                various twilight times, as well as rise/set times   
 * Return value:  0 = sun rises/sets this day, times stored at        
 *                    *trise and *tset.                               
 *               +1 = sun above the specified "horizon" 24 hours.     
 *                    *trise set to time when the sun is at south,    
 *                    minus 12 hours while *tset is set to the south  
 *                    time plus 12 hours. "Day" length = 24 hours     
 *               -1 = sun is below the specified "horizon" 24 hours   
 *                    "Day" length = 0 hours, *trise and *tset are    
 *                    both set to the time when the sun is at south.  
 *                                                                    
 */
int timelib_astro_rise_set_altitude(timelib_time *t_loc, double lon, double lat, double altit, int upper_limb, double *h_rise, double *h_set, timelib_sll *ts_rise, timelib_sll *ts_set)
{
	double  d,  /* Days since 2000 Jan 0.0 (negative before) */
	sr,         /* Solar distance, astronomical units */
	sRA,        /* Sun's Right Ascension */
	sdec,       /* Sun's declination */
	sradius,    /* Sun's apparent radius */
	t,          /* Diurnal arc */
	tsouth,     /* Time when Sun is at south */
	sidtime;    /* Local sidereal time */
	timelib_time *t_utc;
	timelib_sll   timestamp;

	int rc = 0; /* Return cde from function - usually 0 */

	/* Normalize time */
	t_loc->h = 12;
	t_loc->i = t_loc->s = 0;
	timelib_update_ts(t_loc, NULL);

	/* Calculate TS belonging to UTC 00:00 of the current day */
	t_utc = timelib_time_ctor();
	t_utc->y = t_loc->y;
	t_utc->m = t_loc->m;
	t_utc->d = t_loc->d;
	t_utc->h = t_utc->i = t_utc->s = 0;
	timelib_update_ts(t_utc, NULL);

	/* Compute d of 12h local mean solar time */
	timestamp = t_loc->sse;
	d = timelib_ts_to_juliandate(timestamp) - lon/360.0;

	/* Compute local sidereal time of this moment */
	sidtime = astro_revolution(astro_GMST0(d) + 180.0 + lon);

	/* Compute Sun's RA + Decl at this moment */
	astro_sun_RA_dec( d, &sRA, &sdec, &sr );

	/* Compute time when Sun is at south - in hours UT */
	tsouth = 12.0 - astro_rev180(sidtime - sRA) / 15.0;

	/* Compute the Sun's apparent radius, degrees */
	sradius = 0.2666 / sr;

	/* Do correction to upper limb, if necessary */
	if (upper_limb) {
		altit -= sradius;
	}

	/* Compute the diurnal arc that the Sun traverses to reach */
	/* the specified altitude altit: */
	{
		double cost;
		cost = (sind(altit) - sind(lat) * sind(sdec)) / (cosd(lat) * cosd(sdec));
		if (cost >= 1.0) {
			rc = -1;
			t = 0.0;       /* Sun always below altit */

			*ts_rise = *ts_set = t_utc->sse + (tsouth * 3600);
		} else if (cost <= -1.0) {
			rc = +1;
			t = 12.0;      /* Sun always above altit */

			*ts_rise = t_loc->sse - (12 * 3600);
			*ts_set  = t_loc->sse + (12 * 3600);
		} else {
			t = acosd(cost) / 15.0;   /* The diurnal arc, hours */

			/* Store rise and set times - as Unix Timestamp */
			*ts_rise = ((tsouth - t) * 3600) + t_utc->sse;
			*ts_set  = ((tsouth + t) * 3600) + t_utc->sse;

			*h_rise = (tsouth - t);
			*h_set  = (tsouth + t);
		}
	}


	/* Kill temporary time */
	timelib_time_dtor(t_utc);

	return rc;
}

double timelib_ts_to_juliandate(timelib_sll ts)
{
	double tmp;

	tmp = ts;
	tmp /= 86400;
	tmp += 2440587.5;
	tmp -= 2451543;

	return tmp;
}