feat: add GetMeanAnomaly(datetime time.Time) to sun module in @observerly/sidera.

pkg/solar/sun.go

@@ -0,0 +1,49 @@
+/*****************************************************************************************************************/
+
+//	@author		Michael Roberts <michael@observerly.com>
+//	@package	@observerly/sidera
+//	@license	Copyright © 2021-2024 observerly
+
+/*****************************************************************************************************************/
+
+package sun
+
+/*****************************************************************************************************************/
+
+import (
+	"math"
+	"time"
+
+	"github.com/observerly/sidera/pkg/epoch"
+)
+
+/*****************************************************************************************************************/
+
+/*
+the Mean Anomaly of the Sun for a given datetime
+
+The Solar Mean Anomaly is the angle between the perihelion of the Earth's orbit and the current position
+of the Earth along its orbit around the Sun. It is measured in degrees and increases uniformly with time.
+
+The Solar Mean Anomaly is an important concept in solar astronomy, as it is used to calculate the position
+of the Sun in the sky at any given time. By knowing the Solar Mean Anomaly, an observer can determine the
+Sun's position relative to the vernal equinox and calculate the time of sunrise, sunset, and other solar events.
+*/
+func GetMeanAnomaly(datetime time.Time) float64 {
+	// get the Julian Date for the current epoch:
+	JD := epoch.GetJulianDate(datetime)
+
+	// calculate the number of centuries since J2000.0:
+	T := (JD - 2451545.0) / 36525
+
+	// get the Sun's mean anomaly at the current epoch relative to J2000:
+	M := (357.52911 + 35999.05029*T - 0.0001537*math.Pow(T, 2))
+
+	if M < 0 {
+		M += 360
+	}
+
+	return math.Mod(M, 360)
+}
+
+/*****************************************************************************************************************/

pkg/solar/sun_test.go

@@ -0,0 +1,33 @@
+/*****************************************************************************************************************/
+
+//	@author		Michael Roberts <michael@observerly.com>
+//	@package	@observerly/sidera
+//	@license	Copyright © 2021-2024 observerly
+
+/*****************************************************************************************************************/
+
+package sun
+
+/*****************************************************************************************************************/
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+/*****************************************************************************************************************/
+
+// We define a datetime as some arbitrary date and time for testing purposes:
+var datetime time.Time = time.Date(2021, 5, 14, 0, 0, 0, 0, time.UTC)
+
+/*****************************************************************************************************************/
+
+func TestGetSolarMeanAnomaly(t *testing.T) {
+	var got float64 = GetMeanAnomaly(datetime)
+
+	assert.Equal(t, got, 128.66090142411576)
+}
+
+/*****************************************************************************************************************/