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astro.c
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astro.c
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/*
* The MIT License (MIT)
*
* Copyright (c) 2015-2019 Derick Rethans
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
/*
| Algorithms are taken from a public domain source by Paul |
| Schlyter, who wrote this in December 1992 |
*/
#include "timelib.h"
#include <stdio.h>
#include <math.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, timelib_sll *ts_transit)
{
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, old_sse;
int rc = 0; /* Return cde from function - usually 0 */
/* Normalize time */
old_sse = t_loc->sse;
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, for input into
* the algorithm */
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_utc->sse;
d = timelib_ts_to_j2000(timestamp) + 2 - 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));
*ts_transit = t_utc->sse + (tsouth * 3600);
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 and restore original sse */
timelib_time_dtor(t_utc);
t_loc->sse = old_sse;
return rc;
}
double timelib_ts_to_julianday(timelib_sll ts)
{
double tmp;
tmp = (double) ts;
tmp /= (double) 86400;
tmp += (double) 2440587.5;
return tmp;
}
double timelib_ts_to_j2000(timelib_sll ts)
{
return timelib_ts_to_julianday(ts) - 2451545;
}