Merge pull request #79 from observerly/feature/temporal/convertLocalS…

…iderealTimeToGreenwhichSiderealTime

feat: Added convertLocalSiderealTimeToGreenwhichSiderealTime() to temporal module in @observerly/astrometry.

src/temporal.ts

@@ -6,6 +6,38 @@
 
 /*****************************************************************************************************************/
 
-export {}
+import { type GeographicCoordinate } from './common'
+
+/*****************************************************************************************************************/
+
+/**
+ *
+ * convertLocalSiderealTimeToGreenwhichSiderealTime()
+ *
+ * Makes the conversion from Greenwich Sidereal Time to Local Sidereal Time.
+ *
+ * @param LST - a given Local Sidereal Time
+ * @param observer - The geographic coordinate of the observer.
+ * @returns GST - the Greenwich Sidereal Time for the given Local Sidereal Times
+ *
+ */
+export const convertLocalSiderealTimeToGreenwhichSiderealTime = (
+  LST: number,
+  observer: GeographicCoordinate
+): number => {
+  const { longitude } = observer
+
+  let GST = LST - longitude / 15.0
+
+  if (GST < 0) {
+    GST += 24
+  }
+
+  if (GST > 24) {
+    GST -= 24
+  }
+
+  return GST
+}
 
 /*****************************************************************************************************************/

tests/temporal.spec.ts

@@ -0,0 +1,47 @@
+/*****************************************************************************************************************/
+
+// @author         Michael Roberts <[email protected]>
+// @package        @observerly/astrometry/temporal
+// @license        Copyright © 2021-2023 observerly
+
+/*****************************************************************************************************************/
+
+import { describe, expect, it } from 'vitest'
+
+/*****************************************************************************************************************/
+
+import {
+  getLocalSiderealTime,
+  getGreenwhichSiderealTime,
+  convertLocalSiderealTimeToGreenwhichSiderealTime
+} from '../src'
+
+/*****************************************************************************************************************/
+
+// For testing we need to specify a date because most calculations are
+// differential w.r.t a time component. We set it to the author's birthday:
+export const datetime = new Date('2021-05-14T00:00:00.000+00:00')
+
+// For testing we will fix the latitude to be Manua Kea, Hawaii, US
+export const latitude = 19.820611
+
+// For testing we will fix the longitude to be Manua Kea, Hawaii, US:
+export const longitude = -155.468094
+
+/*****************************************************************************************************************/
+
+describe('convertLocalSiderealTimeToGreenwhichSiderealTime', () => {
+  it('should be defined', () => {
+    expect(convertLocalSiderealTimeToGreenwhichSiderealTime).toBeDefined()
+  })
+
+  it('should return the correct Local Sidereal Time for a given Greenwich Sidereal Time', () => {
+    const LST = getLocalSiderealTime(datetime, longitude)
+    const GST = getGreenwhichSiderealTime(datetime)
+    expect(
+      convertLocalSiderealTimeToGreenwhichSiderealTime(LST, { latitude, longitude })
+    ).toBeCloseTo(GST)
+  })
+})
+
+/*****************************************************************************************************************/