private static DateTime SeasonAdjustedEveningTwilight(
double latitude, int day, int year, DateTime sunset)
{
double a = 75 + ((25.60 / 55.0) * Math.Abs(latitude));
double b = 75 + ((2.050 / 55.0) * Math.Abs(latitude));
double c = 75 - ((9.210 / 55.0) * Math.Abs(latitude));
double d = 75 + ((6.140 / 55.0) * Math.Abs(latitude));
double adjustment;
int dyy = PrayerTimes.DaysSinceSolstice(day, year, latitude);
if (dyy < 91)
{
adjustment = a + (b - a) / 91.0 * dyy;
}
else if (dyy < 137)
{
adjustment = b + (c - b) / 46.0 * (dyy - 91);
}
else if (dyy < 183)
{
adjustment = c + (d - c) / 46.0 * (dyy - 137);
}
else if (dyy < 229)
{
adjustment = d + (c - d) / 46.0 * (dyy - 183);
}
else if (dyy < 275)
{
adjustment = c + (b - c) / 46.0 * (dyy - 229);
}
else
{
adjustment = b + (a - b) / 91.0 * (dyy - 275);
}
return(sunset.AddSeconds((int)Math.Round(adjustment * 60.0)));
}
private static DateTime SeasonAdjustedMorningTwilight(
double latitude, int day, int year, DateTime sunrise)
{
double a = 75 + ((28.65 / 55.0) * Math.Abs(latitude));
double b = 75 + ((19.44 / 55.0) * Math.Abs(latitude));
double c = 75 + ((32.74 / 55.0) * Math.Abs(latitude));
double d = 75 + ((48.10 / 55.0) * Math.Abs(latitude));
double adjustment;
int dyy = PrayerTimes.DaysSinceSolstice(day, year, latitude);
if (dyy < 91)
{
adjustment = a + (b - a) / 91.0 * dyy;
}
else if (dyy < 137)
{
adjustment = b + (c - b) / 46.0 * (dyy - 91);
}
else if (dyy < 183)
{
adjustment = c + (d - c) / 46.0 * (dyy - 137);
}
else if (dyy < 229)
{
adjustment = d + (c - d) / 46.0 * (dyy - 183);
}
else if (dyy < 275)
{
adjustment = c + (b - c) / 46.0 * (dyy - 229);
}
else
{
adjustment = b + (a - b) / 91.0 * (dyy - 275);
}
return(sunrise.AddSeconds(-(int)Math.Round(adjustment * 60.0)));
}