/// <summary>
/// Get the sun's right ascension and declination IN RADIANS
/// See the chapter on Solar Coordinates (Chapter 24)</summary>
/// Page 151 onwards
/// This function uses the more accurate method of page 154 onwards.
/// <param name="alpha"></param>
/// <param name="delta"></param>
/// <param name="date">The date for which to get the sun's position</param>
/// <param name="time">The time for which to get the sun's position (zero = midnight, 1.0 = midnight the next day)</param>
public static void GetSunPos(out double alpha, out double delta, DateTime date, double time = 0)
{
double jde = Utils.GetJulianDate(date) + time;
double tau = (jde - 2451545.0) / 365250;
double l, b, r;
// get heliocentric coordinates for earth from VSOP series:
l = VSop87d.SumSeries(VSOPEarth.Along, tau); // radians
b = VSop87d.SumSeries(VSOPEarth.Alat, tau); // radians
r = VSop87d.SumSeries(VSOPEarth.ARad, tau); // astronomical units
// adjust to get geocentric coordinates of the sun:
double sun = l + PI;
double beta = -b;
// get in 0-360 degrees:
sun = Fix2Pi(sun);
// Do the FK5 corrections - page 154 equations 24.9
double T = 10 * tau;
double lamdadash = sun - (1.397 * T + 0.00031 * T * T) * PI / 180.0;
double deltasun = -0.09033 / 3600.0 * PI / 180.0;
double deltabeta = 0.03916 * (Cos(lamdadash) - Sin(lamdadash)) / 3600.0 * PI / 180.0;
sun += deltasun;
beta += deltabeta;
// Get nutations in longitude and obliquity - these are returned
// in RADIANS:
Nutation.GetNutation(out var dpsi, out var depsilon, jde);
sun += dpsi;
// Caculate the correction in the sun's longitude due to abberation:
// Equations 24.11 on page 155 and the long equation on page 156
double deltalambda = GetDeltaLamda(jde); // in RADIANS
double abberation = 0.005775518 * r * deltalambda;
sun -= abberation;
double epsilon0 = GetEpsilon0(jde);
epsilon0 += depsilon;
// get alpha and delta in RADIANS:
alpha = Atan2((Cos(epsilon0) * Sin(sun)),
Cos(sun));
delta = Asin(Sin(epsilon0) * Sin(sun));
}
private static void SumSeries2()
{
var date = new DateTime(2019, 9, 16);
double jde = Utils.GetJulianDate(date);
double tau = (jde - 2451545.0) / 365250;
double l, b, r;
// get heliocentric coordinates for earth from VSOP series:
l = VSop87d.SumSeries(VSOPEarth.Along, tau); // radians
b = VSop87d.SumSeries(VSOPEarth.Alat, tau); // radians
r = VSop87d.SumSeries(VSOPEarth.ARad, tau); // astronomical units
}