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Merge pull request #74 from observerly/feature/transit/doesBodyRiseOrSet
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feat: Added doesBodyRiseOrSet() to transit module in @observerly/astrometry.
michealroberts authored Aug 28, 2023
2 parents f28305c + 2fcbb94 commit 99c77a8
Showing 2 changed files with 81 additions and 2 deletions.
45 changes: 44 additions & 1 deletion src/transit.ts
Original file line number Diff line number Diff line change
@@ -15,7 +15,7 @@ import {
} from './common'
import { convertEquatorialToHorizontal } from './coordinates'

import { getNormalizedInclinationDegree } from './utilities'
import { getNormalizedInclinationDegree, convertDegreesToRadians as radians } from './utilities'

/*****************************************************************************************************************/

@@ -131,3 +131,46 @@ export const isBodyAboveHorizon = (
}

/*****************************************************************************************************************/

/**
*
* doesBodyRiseOrSet()
*
* An object rises or sets if it is above the observer's horizon at the time of observation.
*
* @param observer - The geographic coordinate of the observer.
* @param target - The equatorial or horizontal coordinate of the observed object.
* @returns false if the object never rises or sets for the observer, otherwise returns the Ar and H1 transit parameters.
*
*/
export const doesBodyRiseOrSet = (
observer: GeographicCoordinate,
target: EquatorialCoordinate
): false | Parameters => {
// We only need to consider the latitude of the observer:
const { latitude } = observer

// We only need to consider the declination of the target object:
const { dec } = target

// If |Ar| > 1, the object will never rise or set for the observer.
const Ar = Math.sin(radians(dec)) / Math.cos(radians(latitude))

if (Math.abs(Ar) > 1) {
return false
}

// If |H1| > 1, the object will never rise or set for the observer.
const H1 = Math.tan(radians(latitude)) * Math.tan(radians(dec))

if (Math.abs(H1) > 1) {
return false
}

return {
Ar,
H1
}
}

/*****************************************************************************************************************/
38 changes: 37 additions & 1 deletion tests/transit.spec.ts
Original file line number Diff line number Diff line change
@@ -14,7 +14,8 @@ import {
type EquatorialCoordinate,
isBodyCircumpolar,
isBodyVisible,
isBodyAboveHorizon
isBodyAboveHorizon,
doesBodyRiseOrSet
} from '../src'

/*****************************************************************************************************************/
@@ -249,3 +250,38 @@ describe('isBodyAboveHorizon', () => {
})

/*****************************************************************************************************************/

describe('doesBodyRiseOrSet', () => {
it('should be defined', () => {
expect(doesBodyRiseOrSet).toBeDefined()
})

it('should return transit parameters for a northerm hemisphere object for a postive latitude', () => {
expect(
doesBodyRiseOrSet(
{
latitude,
longitude
},
betelgeuse
)
).toEqual({
Ar: 0.13703603027752978,
H1: 0.04685668461549029
})
})

it('should return false for a nothern hemisphere object for a negative latitude', () => {
expect(
doesBodyRiseOrSet(
{
latitude: -85,
longitude
},
polaris
)
).toBe(false)
})
})

/*****************************************************************************************************************/

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