Merge pull request #22 from jfcarr-astronomy/sun/equation-time

Calculate -> Equation of time

PALib.Tests/PASun.cs

@@ -39,9 +39,13 @@ public void SunriseAndSunset()
 		[Fact]
 		public void MorningAndEveningTwilight()
 		{
-			var result = _paSun.MorningAndEveningTwilight(7, 9, 1979, false, 0, 0, 52, PATwilightType.Astronomical);
+			Assert.Equal((3, 17, 20, 37, "OK"), _paSun.MorningAndEveningTwilight(7, 9, 1979, false, 0, 0, 52, PATwilightType.Astronomical));
+		}
 
-			Assert.Equal((3, 17, 20, 37, "OK"), result);
+		[Fact]
+		public void EquationOfTime()
+		{
+			Assert.Equal((6, 31.52), _paSun.EquationOfTime(27, 7, 2010));
 		}
 	}
 }

PALib/PASun.cs

@@ -217,5 +217,28 @@ public class PASun
 
 			return (amTwilightBeginsHour, amTwilightBeginsMin, pmTwilightEndsHour, pmTwilightEndsMin, status);
 		}
+
+		/// <summary>
+		/// Calculate the equation of time. (The difference between the real Sun time and the mean Sun time.)
+		/// </summary>
+		/// <param name="gwdate_day">Greenwich date (day part)</param>
+		/// <param name="gwdate_month">Greenwich date (month part)</param>
+		/// <param name="gwdate_year">Greenwich date (year part)</param>
+		/// <returns>
+		/// <para>equation_of_time_min -- equation of time (minute part)</para>
+		/// <para>equation_of_time_sec -- equation of time (seconds part)</para>
+		/// </returns>
+		public (double equationOfTimeMin, double equationOfTimeSec) EquationOfTime(double gwdateDay, int gwdateMonth, int gwdateYear)
+		{
+			var sunLongitudeDeg = PAMacros.SunLong(12, 0, 0, 0, 0, gwdateDay, gwdateMonth, gwdateYear);
+			var sunRAHours = PAMacros.DecimalDegreesToDegreeHours(PAMacros.EcRA(sunLongitudeDeg, 0, 0, 0, 0, 0, gwdateDay, gwdateMonth, gwdateYear));
+			var equivalentUTHours = PAMacros.GreenwichSiderealTimeToUniversalTime(sunRAHours, 0, 0, gwdateDay, gwdateMonth, gwdateYear);
+			var equationOfTimeHours = equivalentUTHours - 12;
+
+			var equationOfTimeMin = PAMacros.DecimalHoursMinute(equationOfTimeHours);
+			var equationOfTimeSec = PAMacros.DecimalHoursSecond(equationOfTimeHours);
+
+			return (equationOfTimeMin, equationOfTimeSec);
+		}
 	}
 }

README.md

@@ -43,7 +43,7 @@ If you're interested in this topic, please buy the book! It provides far more de
 - [x] Calculate -> Sun's distance and angular size
 - [x] Calculate -> Local sunrise and sunset
 - [x] Calculate -> Morning and evening twilight
-- [ ] Calculate -> Equation of time
+- [x] Calculate -> Equation of time
 - [ ] Calculate -> Solar elongation
 
 ### Planets