From b4b1fd9d519a5e2a29cfdf5c042c5b61ae950109 Mon Sep 17 00:00:00 2001
From: "Michael J. Roberts" <84131395+michealroberts@users.noreply.github.com>
Date: Mon, 28 Aug 2023 16:29:39 +0100
Subject: [PATCH] feat: Added
 convertLocalSiderealTimeToGreenwhichSiderealTime() to temporal module in
 @observerly/astrometry.

feat: Added convertLocalSiderealTimeToGreenwhichSiderealTime() to temporal module in @observerly/astrometry. Includes associated test suite and expected API output.
---
 src/temporal.ts        | 34 +++++++++++++++++++++++++++++-
 tests/temporal.spec.ts | 47 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+), 1 deletion(-)
 create mode 100644 tests/temporal.spec.ts

diff --git a/src/temporal.ts b/src/temporal.ts
index 9d3392d..ac4e8de 100644
--- a/src/temporal.ts
+++ b/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
+}
 
 /*****************************************************************************************************************/
diff --git a/tests/temporal.spec.ts b/tests/temporal.spec.ts
new file mode 100644
index 0000000..2de8cb7
--- /dev/null
+++ b/tests/temporal.spec.ts
@@ -0,0 +1,47 @@
+/*****************************************************************************************************************/
+
+// @author         Michael Roberts <michael@observerly.com>
+// @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)
+  })
+})
+
+/*****************************************************************************************************************/