/// <summary>Calculates time of Equinox and Solstice.
/// </summary>
/// <param name="year">Year to calculate for</param>
/// <param name="season">Event to calculate</param>
/// <returns>Date and time event occurs as a fractional Julian Day.</returns>
private static double EquinoxSolstice(double year, EquinoxSolsticeEnum season)
{
double julianEphemerisDay = 0;
if (year >= 1000)
{
var y = (Math.Floor(year) - 2000) / 1000;
switch (season)
{
case EquinoxSolsticeEnum.VernalEquinox:
julianEphemerisDay = 2451623.80984
+ (365242.37404 * y)
+ (0.05169 * (y * y))
- (0.00411 * (y * y * y))
- (0.00057 * (y * y * y * y));
break;
case EquinoxSolsticeEnum.SummerSolstice:
julianEphemerisDay = 2451716.56767
+ (365241.62603 * y)
+ (0.00325 * (y * y))
- (0.00888 * (y * y * y))
- (0.00030 * (y * y * y * y));
break;
case EquinoxSolsticeEnum.AutumnalEquinox:
julianEphemerisDay = 2451810.21715
+ (365242.01767 * y)
+ (0.11575 * (y * y))
- (0.00337 * (y * y * y))
- (0.00078 * (y * y * y * y));
break;
case EquinoxSolsticeEnum.WinterSolstice:
julianEphemerisDay = 2451900.05952
+ (365242.74049 * y)
+ (0.06223 * (y * y))
- (0.00823 * (y * y * y))
- (0.00032 * (y * y * y * y));
break;
}
}
else
{
var y = Math.Floor(year) / 1000;
switch (season)
{
case EquinoxSolsticeEnum.VernalEquinox:
julianEphemerisDay = 1721139.29189
+ (365242.13740 * y)
+ (0.06134 * (y * y))
- (0.00111 * (y * y * y))
- (0.00071 * (y * y * y * y));
break;
case EquinoxSolsticeEnum.SummerSolstice:
julianEphemerisDay = 1721233.25401
+ (365241.72562 * y)
+ (0.05323 * (y * y))
- (0.00907 * (y * y * y))
- (0.00025 * (y * y * y * y));
break;
case EquinoxSolsticeEnum.AutumnalEquinox:
julianEphemerisDay = 1721325.70455
+ (365242.49558 * y)
+ (0.11677 * (y * y))
- (0.00297 * (y * y * y))
- (0.00074 * (y * y * y * y));
break;
case EquinoxSolsticeEnum.WinterSolstice:
julianEphemerisDay = 1721414.39987
+ (365242.88257 * y)
+ (0.00769 * (y * y))
- (0.00933 * (y * y * y))
- (0.00006 * (y * y * y * y));
break;
}
}
var julianCenturies = (julianEphemerisDay - 2451545.0) / 36525;
var w = (35999.373 * julianCenturies) - 2.47;
var lambda = 1 + (0.0334 * Math.Cos(w * Deg2Radian))
+ (0.0007 * Math.Cos(2 * w * Deg2Radian));
double Calc(int factor, double v1, double v2)
=> factor *Math.Cos((Deg2Radian *v1) + (Deg2Radian * (v2 *julianCenturies)));
// sum of periodic terms
var sum = Calc(485, 324.96, 1934.136)
+ Calc(203, 337.23, 32964.467)
+ Calc(199, 342.08, 20.186)
+ Calc(182, 27.85, 445267.112)
+ Calc(156, 73.14, 45036.886)
+ Calc(136, 171.52, 22518.443)
+ Calc(77, 222.54, 65928.934)
+ Calc(74, 296.72, 3034.906)
+ Calc(70, 243.58, 9037.513)
+ Calc(58, 119.81, 33718.147)
+ Calc(52, 297.17, 150.678)
+ Calc(50, 21.02, 2281.226)
+ Calc(45, 247.54, 29929.562)
+ Calc(44, 325.15, 31555.956)
+ Calc(29, 60.93, 4443.417)
+ Calc(28, 155.12, 67555.328)
+ Calc(17, 288.79, 4562.452)
+ Calc(16, 198.04, 62894.029)
+ Calc(14, 199.76, 31436.921)
+ Calc(12, 95.39, 14577.848)
+ Calc(12, 287.11, 31931.756)
+ Calc(12, 320.81, 34777.259)
+ Calc(9, 227.73, 1222.114)
+ Calc(8, 15.45, 16859.074);
return(julianEphemerisDay + (0.00001 * sum / lambda));
}
/// <summary>Get the date the equinox or solstice occurs.</summary>
/// <param name="year">The year to get the value for</param>
/// <param name="season">Equinox or solstice to calculate for.</param>
/// <returns>Date and time event occurs.</returns>
private static DateTime GetEquinoxSolstice(int year, EquinoxSolsticeEnum season)
{
return(Julian2Date(EquinoxSolstice(year, season)));
}
