private double ZenithDegrees(ZenithTypes type)
{
double degrees;
switch (type)
{
case ZenithTypes.Official:
degrees = 90.5;
break;
case ZenithTypes.Civil:
degrees = 96;
break;
case ZenithTypes.Nautical:
degrees = 102;
break;
case ZenithTypes.Astronomical:
degrees = 108;
break;
case ZenithTypes.Custom:
degrees = 95;
break;
default:
throw new Exception();
}
return(degrees);
}
public DateTime Sunset(DateTime date, ZenithTypes zenithType)
{
var timeZone = utcOffset.TotalHours;
var zenith = new Angle(ZenithDegrees(zenithType));
//1: calculate the day of the year
var N = date.DayOfYear;
//2: convert the longitude to hour value and calculate an approximate time
//setting time is desired
var t = N + (18 - longitude.Hours) / 24;
//3: calculate the Sun's mean anomaly
var M = new Angle(0.9856 * t - 3.289);
//4: calculate the Sun's true longitude
var L = Angle.Normalize360(M + 1.916 * M.Sin + 0.020 * (2.0 * M).Sin + 282.634);
//5: calculate the Sun's right ascension
var RA = Angle.Normalize360(Angle.Atan(0.91764 * L.Tan));
//right ascension value needs to be in the same quadrant as L
var Lquadrant = Math.Floor(L.Degrees / 90.0) * 90.0;
var RAquadrant = Math.Floor(RA.Degrees / 90.0) * 90.0;
RA.Degrees = RA.Degrees - RAquadrant + Lquadrant;
//6: calculate the Sun's declination
var sinDec = 0.39782 * L.Sin;
var cosDec = Angle.Asin(sinDec).Cos;
//7 calculate the Sun's local hour angle
var cosH = (zenith.Cos - (sinDec * latitude.Sin)) / (cosDec * latitude.Cos);
if (cosH > 1)
{
//the sun never rises on this location (on the specified date)
return(new DateTime(0));
}
//setting time is desired
var H = Angle.Acos(cosH);
//8: calculate local mean time of rising
var T = H.Hours + RA.Hours - (0.06571 * t) - 6.622;
//9: adjust back to UTC
var UT = (T - longitude.Hours + 24) % 24;
//10: convert UT value to local time zone
var localT = UT + timeZone;
var hours = Math.Floor(localT);
var minutes = Math.Floor((localT - hours) * 60);
var seconds = (localT - hours - (minutes / 60)) * 3600;
return(new DateTime(date.Year, date.Month, date.Day, (int)hours, (int)minutes, (int)seconds));
}
