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/*
SUNRISET.C - computes Sun rise/set times, start/end of twilight, and
the length of the day at any date and latitude
Written as DAYLEN.C, 1989-08-16
Modified to SUNRISET.C, 1992-12-01
(c) Paul Schlyter, 1989, 1992
Released to the public domain by Paul Schlyter, December 1992
*/
#include <stdio.h>
#include <math.h>
#include "sunriset.h"
/* The "workhorse" function for sun rise/set times */
int __sunriset__( int year, int month, int day, double lon, double lat,
double altit, int upper_limb, double *trise, double *tset )
/***************************************************************************/
/* Note: year,month,date = calendar date, 1801-2099 only. */
/* 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 comupte */
/* 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. */
/* */
/**********************************************************************/
{
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 */
int rc = 0; /* Return cde from function - usually 0 */
/* Compute d of 12h local mean solar time */
d = days_since_2000_Jan_0(year,month,day) + 0.5 - lon/360.0;
/* Compute local sideral time of this moment */
sidtime = revolution( GMST0(d) + 180.0 + lon );
/* Compute Sun's RA + Decl at this moment */
sun_RA_dec( d, &sRA, &sdec, &sr );
/* Compute time when Sun is at south - in hours UT */
tsouth = 12.0 - 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 altitide 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 */
else if ( cost <= -1.0 )
rc = +1, t = 12.0; /* Sun always above altit */
else
t = acosd(cost)/15.0; /* The diurnal arc, hours */
}
/* Store rise and set times - in hours UT */
*trise = tsouth - t;
*tset = tsouth + t;
return rc;
} /* __sunriset__ */
/* The "workhorse" function */
double __daylen__( int year, int month, int day, double lon, double lat,
double altit, int upper_limb )
/**********************************************************************/
/* Note: year,month,date = calendar date, 1801-2099 only. */
/* Eastern longitude positive, Western longitude negative */
/* Northern latitude positive, Southern latitude negative */
/* The longitude value is not critical. Set it to the correct */
/* longitude if you're picky, otherwise set to to, say, 0.0 */
/* The latitude however IS critical - be sure to get it correct */
/* 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 day length */
/* and to zero when computing day+twilight length. */
/**********************************************************************/
{
double d, /* Days since 2000 Jan 0.0 (negative before) */
obl_ecl, /* Obliquity (inclination) of Earth's axis */
sr, /* Solar distance, astronomical units */
slon, /* True solar longitude */
sin_sdecl, /* Sine of Sun's declination */
cos_sdecl, /* Cosine of Sun's declination */
sradius, /* Sun's apparent radius */
t; /* Diurnal arc */
/* Compute d of 12h local mean solar time */
d = days_since_2000_Jan_0(year,month,day) + 0.5 - lon/360.0;
/* Compute obliquity of ecliptic (inclination of Earth's axis) */
obl_ecl = 23.4393 - 3.563E-7 * d;
/* Compute Sun's position */
sunpos( d, &slon, &sr );
/* Compute sine and cosine of Sun's declination */
sin_sdecl = sind(obl_ecl) * sind(slon);
cos_sdecl = sqrt( 1.0 - sin_sdecl * sin_sdecl );
/* 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 altitide altit: */
{
double cost;
cost = ( sind(altit) - sind(lat) * sin_sdecl ) /
( cosd(lat) * cos_sdecl );
if ( cost >= 1.0 )
t = 0.0; /* Sun always below altit */
else if ( cost <= -1.0 )
t = 24.0; /* Sun always above altit */
else t = (2.0/15.0) * acosd(cost); /* The diurnal arc, hours */
}
return t;
} /* __daylen__ */
/* This function computes the Sun's position at any instant */
void sunpos( double d, double *lon, double *r )
/******************************************************/
/* 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. */
/******************************************************/
{
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 = 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 */
}
void sun_RA_dec( double d, double *RA, double *dec, double *r )
{
double lon, obl_ecl;
double xs, ys, zs;
double xe, ye, ze;
/* Compute Sun's ecliptical coordinates */
sunpos( d, &lon, r );
/* Compute ecliptic rectangular coordinates */
xs = *r * cosd(lon);
ys = *r * sind(lon);
zs = 0; /* because the Sun is always in the ecliptic plane! */
/* 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 */
xe = xs;
ye = ys * cosd(obl_ecl);
ze = ys * sind(obl_ecl);
/* Convert to spherical coordinates */
*RA = atan2d( ye, xe );
*dec = atan2d( ze, sqrt(xe*xe + ye*ye) );
} /* sun_RA_dec */
/******************************************************************/
/* 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 )
double revolution( double x )
/*****************************************/
/* Reduce angle to within 0..360 degrees */
/*****************************************/
{
return( x - 360.0 * floor( x * INV360 ) );
} /* revolution */
double rev180( double x )
/*********************************************/
/* Reduce angle to within -180..+180 degrees */
/*********************************************/
{
return( x - 360.0 * floor( x * INV360 + 0.5 ) );
} /* revolution */
/*******************************************************************/
/* This function computes GMST0, the Greenwhich 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 generelized 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) */
/* */
/*******************************************************************/
double 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 = revolution( ( 180.0 + 356.0470 + 282.9404 ) +
( 0.9856002585 + 4.70935E-5 ) * d );
return sidtim0;
} /* GMST0 */
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