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#ifndef lint |
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static const char RCSid[] = "$Id: sun.c,v 2.6 2013/08/09 16:51:15 greg Exp $"; |
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#endif |
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/* |
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* SOLAR CALCULATIONS |
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* |
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* 3/31/87 |
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* |
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* Michalsky algorithm added October 2019: |
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* "The Astronomical Almanac's Algorithm for Approximate |
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* Solar Position (1950-2050)" by Joseph J. Michalsky, |
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* published in 1988 in Solar Energy, Vol. 40, No. 3. |
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* Also added correction to sdec() (365 was 368 originally) |
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* |
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*/ |
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|
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#include <math.h> |
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#include "sun.h" |
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|
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#ifdef M_PI |
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#define PI M_PI |
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#else |
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#define PI 3.14159265358979323846 |
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#endif |
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#undef DEG |
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#define DEG (PI/180.) |
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|
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|
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double s_latitude = 0.66; /* site latitude (radians north of equator) */ |
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double s_longitude = 2.13; /* site longitude (radians west of Greenwich) */ |
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double s_meridian = 120.*DEG; /* standard meridian (radians west) */ |
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|
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|
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int |
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jdate( /* Julian date (days into year) */ |
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int month, |
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int day |
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) |
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{ |
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static short mo_da[12] = {0,31,59,90,120,151,181,212,243,273,304,334}; |
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|
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return(mo_da[month-1] + day); |
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} |
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|
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|
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double |
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stadj( /* solar time adjustment from Julian date */ |
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int jd |
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) |
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{ |
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return( 0.170 * sin( (4.*PI/373.) * (jd - 80) ) - |
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0.129 * sin( (2.*PI/355.) * (jd - 8) ) + |
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(12./PI) * (s_meridian - s_longitude) ); |
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} |
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|
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|
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double |
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sdec( /* solar declination angle from Julian date */ |
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int jd |
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) |
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{ |
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return( 0.4093 * sin( (2.*PI/365.) * (jd - 81) ) ); |
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} |
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|
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|
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double |
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salt( /* solar altitude from solar declination and solar time */ |
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double sd, |
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double st |
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) |
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{ |
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return( asin( sin(s_latitude) * sin(sd) - |
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cos(s_latitude) * cos(sd) * cos(st*(PI/12.)) ) ); |
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} |
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|
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|
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double |
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sazi( /* solar azimuth from solar declination and solar time */ |
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double sd, |
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double st |
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) |
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{ |
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return( -atan2( cos(sd)*sin(st*(PI/12.)), |
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-cos(s_latitude)*sin(sd) - |
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sin(s_latitude)*cos(sd)*cos(st*(PI/12.)) ) ); |
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} |
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|
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|
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/****************** More accurate Michalsky algorithm ****************/ |
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|
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|
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/* circle normalization */ |
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static double |
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norm_cir(double r, const double p) |
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{ |
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while (r < 0) r += p; |
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while (r >= p) r -= p; |
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return(r); |
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} |
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|
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|
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/* Almanac Julian date relative to noon UT on Jan 1, 2000 (fractional days) */ |
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double |
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mjdate(int year, int month, int day, double hour) |
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{ |
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int jd = jdate(month, day); |
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jd += (month > 2) & !(year&3); |
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jd += (year - 1949)*365 + (year - 1949)/4; |
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hour += s_meridian*(12./PI); |
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return(jd + hour*(1./24.) + (2432916.5-2451545.)); |
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} |
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|
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|
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/* Solar declination (and solar time) from Almanac Julian date (fractional) */ |
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double |
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msdec(double mjd, double *stp) |
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{ /* Ecliptic coordinates (radians) */ |
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double L = norm_cir(280.460*DEG + 0.9856474*DEG*mjd, 2.*PI); |
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double g = norm_cir(357.528*DEG + 0.9856003*DEG*mjd, 2.*PI); |
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double l = L + 1.915*DEG*sin(g) + 0.020*DEG*sin(2.*g); |
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double ep = 23.439*DEG - 4e-7*DEG*mjd; |
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double sin_l = sin(l); |
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|
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if (stp) { /* solar time requested, also? */ |
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double ra = atan2(sin_l*cos(ep), cos(l)); |
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double utime = 24.*(mjd - floor(mjd)) + 12.; |
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double gmst = 6.697375 + 0.0657098242*mjd + utime; |
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double lmst = gmst - s_longitude*(12./PI); |
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|
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*stp = norm_cir(lmst - ra*(12./PI) + 12., 24.); |
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} |
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return(asin(sin(ep)*sin_l)); /* return solar declination angle */ |
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} |
