long
daylight_seconds(const time_t * timer)
{
double l, d;
long n;
/* convert latitude to radians */
l = __latitude / 206264.806;
d = -solar_declination(timer);
/* partial 'Sunrise Equation' */
d = tan(l) * tan(d);
/* magnitude of d may exceed 1.0 at near solstices */
if (d > 1.0)
d = 1.0;
if (d < -1.0)
d = -1.0;
/* derive hour angle */
d = acos(d);
/* but for atmospheric refraction, this would be d /= M_PI */
d /= 3.112505;
n = ONE_DAY * d;
return n;
}
/* Solar angular elevation at the given location and time.
t: Julian centuries since J2000.0
lat: Latitude of location
lon: Longitude of location
Return: Solar angular elevation in radians */
static double
solar_elevation_from_time(double t, double lat, double lon)
{
/* Minutes from midnight */
double jd = jd_from_jcent(t);
double offset = (jd - round(jd) - 0.5)*1440.0;
double eq_time = equation_of_time(t);
double ha = RAD((720 - offset - eq_time)/4 - lon);
double decl = solar_declination(t);
return elevation_from_hour_angle(lat, decl, ha);
}
/* Time of given apparent solar angular elevation of location on earth.
t: Julian centuries since J2000.0
t_noon: Apparent solar noon in Julian centuries since J2000.0
lat: Latitude of location in degrees
lon: Longtitude of location in degrees
elev: Solar angular elevation in radians
Return: Time difference from mean solar midnight in minutes */
static double
time_of_solar_elevation(double t, double t_noon,
double lat, double lon, double elev)
{
/* First pass uses approximate sunrise to
calculate equation of time. */
double eq_time = equation_of_time(t_noon);
double sol_decl = solar_declination(t_noon);
double ha = hour_angle_from_elevation(lat, sol_decl, elev);
double sol_offset = 720 - 4*(lon + DEG(ha)) - eq_time;
/* Recalculate using new sunrise. */
double t_rise = jcent_from_jd(jd_from_jcent(t) + sol_offset/1440.0);
eq_time = equation_of_time(t_rise);
sol_decl = solar_declination(t_rise);
ha = hour_angle_from_elevation(lat, sol_decl, elev);
sol_offset = 720 - 4*(lon + DEG(ha)) - eq_time;
/* No need to do more iterations */
return sol_offset;
}
