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<a href="/plan9/">Plan 9 from Bell Labs</a>’s /usr/web/sources/contrib/bedo/solar.c


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Copyright © 2009 Alcatel-Lucent. 
Distributed under the
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#include<u.h>
#include<libc.h>

double longitude = 2.0 + 20.0 / 60.0;
const double eccentricity = 0.0167;
const double obliquity = 23.44;
const double deg2rad = 2 * PI / 360;

/* Adjusts a time structure by delta min */
void
correct(Tm *t, double delta)
{
int hours, min, sec;

hours = delta / 60;
min = delta - hours * 60;
sec = (delta - (int)delta) * 60;
t->sec += sec;
if(t->sec >= 60) t->sec -= 60, t->min++;
if(t->sec < 0) t->sec += 60, t->min--;
t->min += min;
if(t->min >= 60) t->min -= 60, t->hour++;
if(t->min < 0) t->min += 60, t->hour--;
t->hour += hours;
if(t->hour >= 24) t->hour -= 24;
if(t->hour < 0) t->hour += 24;
}

double
dsin(double x)
{
return sin(x * deg2rad);
}

double
dcos(double x)
{
return cos(x * deg2rad);
}

double
dtan(double x)
{
return tan(x * deg2rad);
}

double
datan(double x)
{
return atan(x) / deg2rad;
}

/* Hex formatter */
#pragma varargck type "H" Tm*
int
hexfmt(Fmt *f)
{
int hours, min, sec;
uint time;
Tm *t;

t = va_arg(f->args, Tm *);
time = 0.5+(float)0x10000 * ((float)t->hour + (float)t->min / 60.0 + (float)t->sec / (60.0 * 60.0)) / 24.0;
hours = time / 0x1000;
min = time % 0x1000 / 0x10;
sec = time % 0x10;
return fmtprint(f, "%01X_%02X_%01X", hours, min, sec);
}

/* Duodecimal formatter */
#pragma varargck type "D" Tm*
int
duofmt(Fmt *f)
{
Tm *t;
t = va_arg(f->args, Tm*);
return fmtprint(f, "%02d:%02d:%02d", t->hour, t->min, t->sec);
}

void
usage(void)
{
fprint(2, "%s: [-l longitude] [-x] [-f]", argv0);
exits("usage");
}

void
main(int argc, char *argv[])
{
Tm *t;
int hex, follow;
char *fmt;

fmt = "%D\n";
hex = 0;
follow = 0;

double w, a, b, c, eot;

ARGBEGIN{
case 'l':
longitude = atof(EARGF(usage()));
break;
case 'x':
fmt = "%H\n";
hex++;
break;
case 'f':
follow++;
break;
case 'h':
default:
usage();
}ARGEND;

if(argc != 0)
usage();

fmtinstall('H', hexfmt);
fmtinstall('D', duofmt);

beginning:
/* Get local mean time */
t = gmtime(time(0));
correct(t, longitude * 4.0);

/* Calculate equation of time */
w = 360 / 365.24; // Mean angular orbit velocity
a = w * (t->yday + 10); // angular orbital distance from solstice (10 days befor jan 1)
b = a + (360 / PI) * eccentricity * dsin(w * (t->yday - 2)); // angular deflection of earth
c = (a - datan(dtan(b) / dcos(obliquity))) / 180; // Difference between mean and corrected angles
eot = 720 * (c - floor(c + 0.5));

/* Alternative formulation */
/* w = 2 * PI * (t->yday - 1) / 365.242;
eot = 0.258 * cos(w) - 7.416 * sin(w) - 3.648 * cos(2 * w) - 9.228 * sin(2 * w);*/

//print("%f\n", eot);
correct(t, -eot);
print(fmt, t);

if(follow){
sleep(hex ? (1000 / 0.76) : 1000);
goto beginning;
}

exits(0);
}

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