private DateTimeOffset CalculateSunriseSunset(GeoLocation location, DateTimeOffset instant,
bool isSunrise)
{
// Based on:
// https://github.com/ceeK/Solar/blob/9d8ed80a3977c97d7a2014ef28b129ec80c52a70/Solar/Solar.swift
// Copyright (c) 2016 Chris Howell (MIT License)
// TODO: this is not 100% accurate
// Convert longitude to hour value and calculate an approximate time
var lngHours = location.Longitude / 15;
var timeApproxHours = isSunrise ? 6 : 18;
var timeApproxDays = instant.DayOfYear + (timeApproxHours - lngHours) / 24;
// Calculate the Sun's mean anomaly
var sunMeanAnomaly = 0.9856 * timeApproxDays - 3.289;
// Calculate the Sun's true longitude
var sunLng = sunMeanAnomaly + 282.634 +
1.916 * Math.Sin(UnitConversion.DegreesToRadians(sunMeanAnomaly)) +
0.020 * Math.Sin(2 * UnitConversion.DegreesToRadians(sunMeanAnomaly));
sunLng %= 360; // wrap [0;360)
// Calculate the Sun's right ascension
var sunRightAsc = UnitConversion.RadiansToDegrees(Math.Atan(0.91764 * Math.Tan(UnitConversion.DegreesToRadians(sunLng))));
sunRightAsc %= 360; // wrap [0;360)
// Right ascension value needs to be in the same quadrant as true longitude
var sunLngQuad = Math.Floor(sunLng / 90) * 90;
var sunRightAscQuad = Math.Floor(sunRightAsc / 90) * 90;
var sunRightAscHours = sunRightAsc + (sunLngQuad - sunRightAscQuad);
sunRightAscHours /= 15;
// Calculate Sun's declination
var sinDec = 0.39782 * Math.Sin(UnitConversion.DegreesToRadians(sunLng));
var cosDec = Math.Cos(Math.Asin(sinDec));
// Calculate the Sun's local hour angle
const double zenith = 90.83; // official sunrise/sunset
var sunLocalHoursCos =
(Math.Cos(UnitConversion.DegreesToRadians(zenith)) - sinDec * Math.Sin(UnitConversion.DegreesToRadians(location.Latitude))) /
(cosDec * Math.Cos(UnitConversion.DegreesToRadians(location.Latitude)));
var sunLocalHours = isSunrise
? 360 - UnitConversion.RadiansToDegrees(Math.Acos(sunLocalHoursCos))
: UnitConversion.RadiansToDegrees(Math.Acos(sunLocalHoursCos));
sunLocalHours /= 15;
// Calculate local mean time
var meanTime = sunLocalHours + sunRightAscHours - 0.06571 * timeApproxDays - 6.622;
return instant.ResetTimeOfDay() + TimeSpan.FromHours(meanTime);
}
