Merge pull request #49 from observerly/feature/moon/getLunarAnnualEqu…

…ationCorrection

feat: Added getLunarAnnualEquationCorrection() to moon module in @observerly/astrometry.

src/moon.ts

@@ -8,6 +8,20 @@
 
 import { getJulianDate } from './epoch'
 
+import { getSolarMeanAnomaly } from './sun'
+
+import { convertDegreesToRadians as radians } from './utilities'
+
+/*****************************************************************************************************************/
+
+export const getLunarAnnualEquationCorrection = (datetime: Date): number => {
+  // Correct for the Sun's mean anomaly:
+  const M = radians(getSolarMeanAnomaly(datetime))
+
+  // Get the annual equation correction:
+  return 0.1858 * Math.sin(M)
+}
+
 /*****************************************************************************************************************/
 
 /**

tests/moon.spec.ts

@@ -11,6 +11,7 @@ import { describe, expect, it } from 'vitest'
 /*****************************************************************************************************************/
 
 import {
+  getLunarAnnualEquationCorrection,
   getLunarMeanAnomaly,
   getLunarMeanGeometricLongitude,
   getLunarMeanEclipticLongitude
@@ -24,6 +25,19 @@ export const datetime = new Date('2021-05-14T00:00:00.000+00:00')
 
 /*****************************************************************************************************************/
 
+describe('getLunarAnnualEquationCorrection', () => {
+  it('should be defined', () => {
+    expect(getLunarAnnualEquationCorrection).toBeDefined()
+  })
+
+  it('should return the correct Lunar annual equation correction for the given date', () => {
+    const Ae = getLunarAnnualEquationCorrection(datetime)
+    expect(Ae).toBe(0.14508321025194074)
+  })
+})
+
+/*****************************************************************************************************************/
+
 describe('getLunarMeanAnomaly', () => {
   it('should be defined', () => {
     expect(getLunarMeanAnomaly).toBeDefined()