From 57ac9c203ad38d4084422a4189b8ab97f22d51c2 Mon Sep 17 00:00:00 2001
From: "Michael J. Roberts" <michael@observerly.com>
Date: Wed, 1 May 2024 16:31:25 +0100
Subject: [PATCH] feat: add GetMeanAnomaly(datetime time.Time) to sun module in
 @observerly/sidera.

feat: add GetMeanAnomaly(datetime time.Time) to sun module in @observerly/sidera.
---
 pkg/solar/sun.go      | 49 +++++++++++++++++++++++++++++++++++++++++++
 pkg/solar/sun_test.go | 33 +++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+)
 create mode 100644 pkg/solar/sun.go
 create mode 100644 pkg/solar/sun_test.go

diff --git a/pkg/solar/sun.go b/pkg/solar/sun.go
new file mode 100644
index 0000000..fa14cd2
--- /dev/null
+++ b/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)
+}
+
+/*****************************************************************************************************************/
diff --git a/pkg/solar/sun_test.go b/pkg/solar/sun_test.go
new file mode 100644
index 0000000..e4f77ea
--- /dev/null
+++ b/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)
+}
+
+/*****************************************************************************************************************/