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414 lines
17 KiB
414 lines
17 KiB
4 years ago
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/* +++Date last modified: 05-Jul-1997 */
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/* Updated comments, 05-Aug-2013 */
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/*
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SUNRISET.C - computes Sun rise/set times, start/end of twilight, and
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the length of the day at any date and latitude
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Written as DAYLEN.C, 1989-08-16
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Modified to SUNRISET.C, 1992-12-01
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Split to a header file, 2017-12-10, by Joachim Nilsson
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(c) Paul Schlyter, 1989, 1992
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Released to the public domain by Paul Schlyter, December 1992
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*/
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#include <stdio.h>
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#include <math.h>
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#include "sunriset.h"
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/* A small test program */
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// #ifndef SUNRISET_LIB
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// int main(void)
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// {
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// int year,month,day;
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// double lon, lat;
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// double daylen, civlen, nautlen, astrlen;
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// double rise, set, civ_start, civ_end, naut_start, naut_end,
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// astr_start, astr_end;
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// int rs‘, civ, naut, astr;
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// char buf[80];
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//
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// printf( "Latitude (+ is north) and longitude (+ is east) : " );
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// fgets(buf, 80, stdin);
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// sscanf(buf, "%lf %lf", &lat, &lon );
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//
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// for(;;)
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// {
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// printf( "Input date ( yyyy mm dd ) (ctrl-C exits): " );
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// fgets(buf, 80, stdin);
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// sscanf(buf, "%d %d %d", &year, &month, &day );
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//
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// daylen = day_length(year,month,day,lon,lat);
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// civlen = day_civil_twilight_length(year,month,day,lon,lat);
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// nautlen = day_nautical_twilight_length(year,month,day,lon,lat);
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// astrlen = day_astronomical_twilight_length(year,month,day,
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// lon,lat);
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//
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// printf( "Day length: %5.2f hours\n", daylen );
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// printf( "With civil twilight %5.2f hours\n", civlen );
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// printf( "With nautical twilight %5.2f hours\n", nautlen );
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// printf( "With astronomical twilight %5.2f hours\n", astrlen );
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// printf( "Length of twilight: civil %5.2f hours\n",
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// (civlen-daylen)/2.0);
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// printf( " nautical %5.2f hours\n",
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// (nautlen-daylen)/2.0);
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// printf( " astronomical %5.2f hours\n",
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// (astrlen-daylen)/2.0);
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//
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// rs = sun_rise_set ( year, month, day, lon, lat,
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// &rise, &set );
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// civ = civil_twilight ( year, month, day, lon, lat,
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// &civ_start, &civ_end );
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// naut = nautical_twilight ( year, month, day, lon, lat,
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// &naut_start, &naut_end );
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// astr = astronomical_twilight( year, month, day, lon, lat,
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// &astr_start, &astr_end );
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//
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// printf( "Sun at south %5.2fh UT\n", (rise+set)/2.0 );
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//
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// switch( rs )
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// {
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// case 0:
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// printf( "Sun rises %5.2fh UT, sets %5.2fh UT\n",
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// rise, set );
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// break;
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// case +1:
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// printf( "Sun above horizon\n" );
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// break;
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// case -1:
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// printf( "Sun below horizon\n" );
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// break;
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// }
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//
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// switch( civ )
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// {
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// case 0:
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// printf( "Civil twilight starts %5.2fh, "
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// "ends %5.2fh UT\n", civ_start, civ_end );
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// break;
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// case +1:
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// printf( "Never darker than civil twilight\n" );
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// break;
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// case -1:
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// printf( "Never as bright as civil twilight\n" );
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// break;
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// }
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//
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// switch( naut )
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// {
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// case 0:
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// printf( "Nautical twilight starts %5.2fh, "
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// "ends %5.2fh UT\n", naut_start, naut_end );
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// break;
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// case +1:
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// printf( "Never darker than nautical twilight\n" );
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// break;
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// case -1:
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// printf( "Never as bright as nautical twilight\n" );
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// break;
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// }
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//
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// switch( astr )
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// {
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// case 0:
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// printf( "Astronomical twilight starts %5.2fh, "
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// "ends %5.2fh UT\n", astr_start, astr_end );
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// break;
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// case +1:
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// printf( "Never darker than astronomical twilight\n" );
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// break;
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// case -1:
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// printf( "Never as bright as astronomical twilight\n" );
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// break;
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// }
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// return 0;
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// }
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// }
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// #endif /* SUNRISET_LIB */
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/* The "workhorse" function for sun rise/set times */
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int __sunriset__( int year, int month, int day, double lon, double lat,
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double altit, int upper_limb, double *trise, double *tset )
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/***************************************************************************/
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/* Note: year,month,date = calendar date, 1801-2099 only. */
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/* Eastern longitude positive, Western longitude negative */
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/* Northern latitude positive, Southern latitude negative */
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/* The longitude value IS critical in this function! */
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/* altit = the altitude which the Sun should cross */
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/* Set to -35/60 degrees for rise/set, -6 degrees */
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/* for civil, -12 degrees for nautical and -18 */
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/* degrees for astronomical twilight. */
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/* upper_limb: non-zero -> upper limb, zero -> center */
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/* Set to non-zero (e.g. 1) when computing rise/set */
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/* times, and to zero when computing start/end of */
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/* twilight. */
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/* *rise = where to store the rise time */
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/* *set = where to store the set time */
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/* Both times are relative to the specified altitude, */
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/* and thus this function can be used to compute */
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/* various twilight times, as well as rise/set times */
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/* Return value: 0 = sun rises/sets this day, times stored at */
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/* *trise and *tset. */
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/* +1 = sun above the specified "horizon" 24 hours. */
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/* *trise set to time when the sun is at south, */
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/* minus 12 hours while *tset is set to the south */
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/* time plus 12 hours. "Day" length = 24 hours */
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/* -1 = sun is below the specified "horizon" 24 hours */
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/* "Day" length = 0 hours, *trise and *tset are */
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/* both set to the time when the sun is at south. */
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/* */
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/**********************************************************************/
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{
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double d, /* Days since 2000 Jan 0.0 (negative before) */
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sr, /* Solar distance, astronomical units */
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sRA, /* Sun's Right Ascension */
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sdec, /* Sun's declination */
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sradius, /* Sun's apparent radius */
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t, /* Diurnal arc */
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tsouth, /* Time when Sun is at south */
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sidtime; /* Local sidereal time */
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int rc = 0; /* Return cde from function - usually 0 */
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/* Compute d of 12h local mean solar time */
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d = days_since_2000_Jan_0(year,month,day) + 0.5 - lon/360.0;
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/* Compute the local sidereal time of this moment */
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sidtime = revolution( GMST0(d) + 180.0 + lon );
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/* Compute Sun's RA, Decl and distance at this moment */
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sun_RA_dec( d, &sRA, &sdec, &sr );
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/* Compute time when Sun is at south - in hours UT */
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tsouth = 12.0 - rev180(sidtime - sRA)/15.0;
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/* Compute the Sun's apparent radius in degrees */
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sradius = 0.2666 / sr;
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/* Do correction to upper limb, if necessary */
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if ( upper_limb )
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altit -= sradius;
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/* Compute the diurnal arc that the Sun traverses to reach */
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/* the specified altitude altit: */
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{
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double cost;
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cost = ( sind(altit) - sind(lat) * sind(sdec) ) /
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( cosd(lat) * cosd(sdec) );
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if ( cost >= 1.0 )
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rc = -1, t = 0.0; /* Sun always below altit */
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else if ( cost <= -1.0 )
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rc = +1, t = 12.0; /* Sun always above altit */
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else
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t = acosd(cost)/15.0; /* The diurnal arc, hours */
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}
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/* Store rise and set times - in hours UT */
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*trise = tsouth - t;
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*tset = tsouth + t;
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return rc;
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} /* __sunriset__ */
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/* The "workhorse" function */
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double __daylen__( int year, int month, int day, double lon, double lat,
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double altit, int upper_limb )
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/**********************************************************************/
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/* Note: year,month,date = calendar date, 1801-2099 only. */
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/* Eastern longitude positive, Western longitude negative */
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/* Northern latitude positive, Southern latitude negative */
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/* The longitude value is not critical. Set it to the correct */
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/* longitude if you're picky, otherwise set to to, say, 0.0 */
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/* The latitude however IS critical - be sure to get it correct */
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/* altit = the altitude which the Sun should cross */
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/* Set to -35/60 degrees for rise/set, -6 degrees */
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/* for civil, -12 degrees for nautical and -18 */
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/* degrees for astronomical twilight. */
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/* upper_limb: non-zero -> upper limb, zero -> center */
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/* Set to non-zero (e.g. 1) when computing day length */
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/* and to zero when computing day+twilight length. */
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/**********************************************************************/
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{
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double d, /* Days since 2000 Jan 0.0 (negative before) */
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obl_ecl, /* Obliquity (inclination) of Earth's axis */
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sr, /* Solar distance, astronomical units */
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slon, /* True solar longitude */
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sin_sdecl, /* Sine of Sun's declination */
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cos_sdecl, /* Cosine of Sun's declination */
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sradius, /* Sun's apparent radius */
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t; /* Diurnal arc */
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/* Compute d of 12h local mean solar time */
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d = days_since_2000_Jan_0(year,month,day) + 0.5 - lon/360.0;
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/* Compute obliquity of ecliptic (inclination of Earth's axis) */
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obl_ecl = 23.4393 - 3.563E-7 * d;
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/* Compute Sun's ecliptic longitude and distance */
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sunpos( d, &slon, &sr );
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/* Compute sine and cosine of Sun's declination */
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sin_sdecl = sind(obl_ecl) * sind(slon);
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cos_sdecl = sqrt( 1.0 - sin_sdecl * sin_sdecl );
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/* Compute the Sun's apparent radius, degrees */
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sradius = 0.2666 / sr;
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/* Do correction to upper limb, if necessary */
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if ( upper_limb )
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altit -= sradius;
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/* Compute the diurnal arc that the Sun traverses to reach */
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/* the specified altitude altit: */
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{
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double cost;
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cost = ( sind(altit) - sind(lat) * sin_sdecl ) /
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( cosd(lat) * cos_sdecl );
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if ( cost >= 1.0 )
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t = 0.0; /* Sun always below altit */
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else if ( cost <= -1.0 )
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t = 24.0; /* Sun always above altit */
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else t = (2.0/15.0) * acosd(cost); /* The diurnal arc, hours */
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}
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return t;
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} /* __daylen__ */
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/* This function computes the Sun's position at any instant */
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void sunpos( double d, double *lon, double *r )
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/******************************************************/
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/* Computes the Sun's ecliptic longitude and distance */
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/* at an instant given in d, number of days since */
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/* 2000 Jan 0.0. The Sun's ecliptic latitude is not */
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/* computed, since it's always very near 0. */
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/******************************************************/
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{
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double M, /* Mean anomaly of the Sun */
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w, /* Mean longitude of perihelion */
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/* Note: Sun's mean longitude = M + w */
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e, /* Eccentricity of Earth's orbit */
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E, /* Eccentric anomaly */
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x, y, /* x, y coordinates in orbit */
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v; /* True anomaly */
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/* Compute mean elements */
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M = revolution( 356.0470 + 0.9856002585 * d );
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w = 282.9404 + 4.70935E-5 * d;
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e = 0.016709 - 1.151E-9 * d;
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/* Compute true longitude and radius vector */
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E = M + e * RADEG * sind(M) * ( 1.0 + e * cosd(M) );
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x = cosd(E) - e;
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y = sqrt( 1.0 - e*e ) * sind(E);
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*r = sqrt( x*x + y*y ); /* Solar distance */
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v = atan2d( y, x ); /* True anomaly */
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*lon = v + w; /* True solar longitude */
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if ( *lon >= 360.0 )
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*lon -= 360.0; /* Make it 0..360 degrees */
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}
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void sun_RA_dec( double d, double *RA, double *dec, double *r )
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/******************************************************/
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/* Computes the Sun's equatorial coordinates RA, Decl */
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/* and also its distance, at an instant given in d, */
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/* the number of days since 2000 Jan 0.0. */
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/******************************************************/
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{
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double lon, obl_ecl, x, y, z;
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/* Compute Sun's ecliptical coordinates */
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sunpos( d, &lon, r );
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/* Compute ecliptic rectangular coordinates (z=0) */
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x = *r * cosd(lon);
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y = *r * sind(lon);
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/* Compute obliquity of ecliptic (inclination of Earth's axis) */
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obl_ecl = 23.4393 - 3.563E-7 * d;
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/* Convert to equatorial rectangular coordinates - x is unchanged */
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z = y * sind(obl_ecl);
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y = y * cosd(obl_ecl);
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/* Convert to spherical coordinates */
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*RA = atan2d( y, x );
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*dec = atan2d( z, sqrt(x*x + y*y) );
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} /* sun_RA_dec */
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/******************************************************************/
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/* This function reduces any angle to within the first revolution */
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/* by subtracting or adding even multiples of 360.0 until the */
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/* result is >= 0.0 and < 360.0 */
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/******************************************************************/
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#define INV360 ( 1.0 / 360.0 )
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double revolution( double x )
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/*****************************************/
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/* Reduce angle to within 0..360 degrees */
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||
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/*****************************************/
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||
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{
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||
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|
return( x - 360.0 * floor( x * INV360 ) );
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||
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} /* revolution */
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||
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|
|
||
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double rev180( double x )
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||
|
|
/*********************************************/
|
||
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/* Reduce angle to within +180..+180 degrees */
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||
|
|
/*********************************************/
|
||
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|
{
|
||
|
|
return( x - 360.0 * floor( x * INV360 + 0.5 ) );
|
||
|
|
} /* revolution */
|
||
|
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|
||
|
|
|
||
|
|
/*******************************************************************/
|
||
|
|
/* This function computes GMST0, the Greenwich Mean Sidereal Time */
|
||
|
|
/* at 0h UT (i.e. the sidereal time at the Greenwich 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) */
|
||
|
|
/* */
|
||
|
|
/*******************************************************************/
|
||
|
|
|
||
|
|
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 */
|