From 07cc6e86982fda4c7e0d12bcac171d483286cc2d Mon Sep 17 00:00:00 2001
From: "Michael J. Roberts" <84131395+michealroberts@users.noreply.github.com>
Date: Mon, 28 Aug 2023 15:55:14 +0100
Subject: [PATCH] feat: Added getBodyTransit() to transit module in
 @observerly/astrometry.

feat: Added getBodyTransit() to transit module in @observerly/astrometry. Includes associated test suite and expected API output.
---
 src/transit.ts        | 66 ++++++++++++++++++++++++++++++++++++++++++-
 tests/transit.spec.ts | 32 ++++++++++++++++++---
 2 files changed, 93 insertions(+), 5 deletions(-)

diff --git a/src/transit.ts b/src/transit.ts
index fb4b28d..b4b0fd0 100644
--- a/src/transit.ts
+++ b/src/transit.ts
@@ -15,7 +15,11 @@ import {
 } from './common'
 import { convertEquatorialToHorizontal } from './coordinates'
 
-import { getNormalizedInclinationDegree, convertDegreesToRadians as radians } from './utilities'
+import {
+  getNormalizedInclinationDegree,
+  convertDegreesToRadians as radians,
+  convertRadiansToDegrees as degrees
+} from './utilities'
 
 /*****************************************************************************************************************/
 
@@ -211,3 +215,63 @@ export const doesBodyRiseOrSet = (
 }
 
 /*****************************************************************************************************************/
+
+/**
+ *
+ * getBodyTransit()
+ *
+ * Determines the local sidereal time and azimuthal angle of rise and set for an object.
+ *
+ * @param observer - The geographic coordinate of the observer.
+ * @param target - The equatorial or horizontal coordinate of the observed object.
+ * @returns the transit for the body, or undefined if the body never rises or sets for the observer.
+ *
+ */
+export const getBodyTransit = (
+  observer: GeographicCoordinate,
+  target: EquatorialCoordinate
+): Transit | undefined => {
+  // Convert the right ascension to hours:
+  const ra = target.ra / 15
+
+  // Get the transit parameters:
+  const body = doesBodyRiseOrSet(observer, target)
+
+  if (!body) {
+    return undefined
+  }
+
+  // Extract the transit parameters from the body:
+  const { H1, Ar } = body
+
+  const H2 = degrees(Math.acos(-H1)) / 15
+
+  // Get the azimuthal angle of rise:
+  const R = degrees(Math.acos(Ar))
+
+  // Get the azimuthal angle of set:
+  const S = 360 - R
+
+  // The local sidereal time of rise:
+  let LSTr = 24 + ra - H2
+
+  if (LSTr > 24) {
+    LSTr -= 24
+  }
+
+  // The local sidereal time of set:
+  let LSTs = ra + H2
+
+  if (LSTs > 24) {
+    LSTs -= 24
+  }
+
+  return {
+    LSTr,
+    LSTs,
+    R,
+    S
+  }
+}
+
+/*****************************************************************************************************************/
diff --git a/tests/transit.spec.ts b/tests/transit.spec.ts
index 7da6a6a..9106f5d 100644
--- a/tests/transit.spec.ts
+++ b/tests/transit.spec.ts
@@ -15,7 +15,8 @@ import {
   isBodyCircumpolar,
   isBodyVisible,
   isBodyAboveHorizon,
-  doesBodyRiseOrSet
+  doesBodyRiseOrSet,
+  getBodyTransit
 } from '../src'
 
 /*****************************************************************************************************************/
@@ -34,7 +35,7 @@ export const longitude = -155.468094
 const polaris: EquatorialCoordinate = { ra: 37.95456, dec: 89.264108 }
 
 // For testing
-const betelgeuse: EquatorialCoordinate = { ra: 5.919529, dec: 7.407064 }
+const betelgeuse: EquatorialCoordinate = { ra: 88.7929583, dec: 7.4070639 }
 
 // For testings
 const sigmaOctantis: EquatorialCoordinate = { ra: 21.07875, dec: -88.9569444 }
@@ -266,8 +267,8 @@ describe('doesBodyRiseOrSet', () => {
         betelgeuse
       )
     ).toEqual({
-      Ar: 0.13703603027752978,
-      H1: 0.04685668461549029
+      Ar: 0.13703602843777568,
+      H1: 0.04685668397579211
     })
   })
 
@@ -285,3 +286,26 @@ describe('doesBodyRiseOrSet', () => {
 })
 
 /*****************************************************************************************************************/
+
+describe('getBodyTransit', () => {
+  it('should be defined', () => {
+    expect(getBodyTransit).toBeDefined()
+  })
+
+  it('should return transit parameters for a northerm hemisphere object for a postive latitude', () => {
+    const { LSTr, LSTs, R, S } = getBodyTransit(
+      {
+        latitude,
+        longitude
+      },
+      betelgeuse
+    )
+
+    expect(LSTr).toBe(23.740485646638913)
+    expect(LSTs).toBe(12.098575460027751)
+    expect(R).toBeCloseTo(82.12362992591511)
+    expect(S).toBeCloseTo(277.8763700740849)
+  })
+})
+
+/*****************************************************************************************************************/