public static DateTime GetDateTimeOfSolarTerm(int termIndex, int year)
{
int[] trialMonths =
{ 3, 4, 4, 5, 5, 6,
6, 7, 7, 8, 8, 9,
9, 10, 10, 11, 11, 12,
12, 1, 1, 2, 2, 3 };
int[] trialDays =
{ 21, 5, 20, 6, 21, 6,
21, 7, 23, 7, 23, 8,
23, 8, 23, 7, 22, 7,
22, 6, 21, 4, 19, 5 };
double desiredSunLong = termIndex * (Math.PI / 12);
DateTime estimatedDateTime = new DateTime(year, trialMonths[termIndex], trialDays[termIndex]);
double currentSunLong = Sun.GetSunLongitudeAtJulianDate(
estimatedDateTime.UniversalDateTimeToJulianDate());
currentSunLong = Sun.GetSunLongitudeAtJulianDate(
estimatedDateTime.UniversalDateTimeToJulianDate());
var error = (currentSunLong - desiredSunLong).ToNormalizedArc();
var direction = 1;
if (error > Math.PI)
{
direction = -1;
}
double resolution = 1; // days
double previousSunLong = currentSunLong;
var count = 0;
do
{
estimatedDateTime = estimatedDateTime.AddDays(resolution * direction);
currentSunLong = Sun.GetSunLongitudeAtJulianDate(estimatedDateTime.UniversalDateTimeToJulianDate());
double error1 = (currentSunLong - desiredSunLong).ToNormalizedArc();
double error2 = (desiredSunLong + 2 * Math.PI - currentSunLong).ToNormalizedArc();
if (error1 > error && error2 > error)
{
direction = direction * (-1);
resolution = resolution / 2;
}
error = error1;
if (error2 > error1)
{
error = error1;
}
else
{
error = error2;
}
count++;
} while (resolution > (1f / 86400) && Math.Abs(error) > 0.00001);
return(estimatedDateTime);
}