/* Calculate the position of the sun at a given time. pages 89-91 */
void
sun_position (time_t unix_time, gdouble *lat, gdouble *lon)
{
gdouble jd, D, N, M, E, x, v, lambda;
gdouble ra, dec;
jd = unix_time_to_julian_date (unix_time);
/* Calculate number of days since the epoch */
D = jd - EPOCH;
N = D*360/365.242191;
/* normalize to 0 - 360 degrees */
NORMALIZE (N);
/* Step 4: */
M = N + EPSILON_G - MU_G;
NORMALIZE (M);
/* Step 5: convert to radians */
M = DEG_TO_RADS (M);
/* Step 6: */
E = solve_keplers_equation (ECCENTRICITY, M);
/* Step 7: */
x = sqrt ((1 + ECCENTRICITY)/(1 - ECCENTRICITY)) * tan (E/2);
/* Step 8, 9 */
v = 2 * RADS_TO_DEG (atan (x));
NORMALIZE (v);
/* Step 10 */
lambda = v + MU_G;
NORMALIZE (lambda);
/* convert the ecliptic longitude to right ascension and declination */
ecliptic_to_equatorial (DEG_TO_RADS (lambda), 0.0, &ra, &dec);
ra = ra - (G_PI/12) * greenwich_sidereal_time (unix_time);
ra = RADS_TO_DEG (ra);
dec = RADS_TO_DEG (dec);
NORMALIZE (ra);
NORMALIZE (dec);
*lat = dec;
*lon = ra;
}
/*
* compute ecliptic longitude of sun (in radians)
* (after duffett-smith, section 47)
*/
static double sun_ecliptic_longitude(time_t ssue)
/* seconds since unix epoch */
{
double D, N;
double M_sun, E;
double v;
D = DaysSinceEpoch(ssue);
N = RadsPerDay * D;
M_sun = mean_sun(D);
E = solve_keplers_equation(M_sun);
v = 2 * atan(sqrt((1 + Eccentricity) / (1 - Eccentricity)) *
tan(E / 2));
return (v + OmegaBar_g);
}
/*
* compute ecliptic longitude of sun (in radians)
* (after duffett-smith, section 47)
*/
static double sun_ecliptic_longitude(time_t ssue)
//time_t ssue; /* seconds since unix epoch */
{
double D, N;
double M_sun, E;
double v;
D = DaysSinceEpoch(ssue);
N = RadsPerDay * D;
N = fmod(N, TWOPI);
if (N < 0) N += TWOPI;
M_sun = N + Epsilon_g - OmegaBar_g;
if (M_sun < 0) M_sun += TWOPI;
E = solve_keplers_equation(M_sun);
v = 2 * atan(sqrt((1+Eccentricity)/(1-Eccentricity)) * tan(E/2));
return (v + OmegaBar_g);
}
