[AstronomicalConstant] Calculations off by ~4.5hrs

[mannylopez]

Looks like the major moon phases have a ~4.5hr margin of error around midnight.

The following notes show what the lib/TinyMoon package show as the major moon phase, and the pst and utc columns show what timeanddate.com show for the actual moon phase.

• PST
• UTC

I was using this to test

func test_debug() {
    let pacificTimeZone = TinyMoon.TimeZoneOption.createTimeZone(timeZone: .pacific)
    let utcTimeZone = TinyMoon.TimeZoneOption.createTimeZone(timeZone: .utc)

//    let month = MonthTestHelper.Month.february
//
//    MoonTestHelper.prettyPrintMoonCalendar(month: month, year: 2024, timeZone: pacificTimeZone)
//    MoonTestHelper.prettyPrintMoonCalendar(month: month, year: 2024, timeZone: utcTimeZone)

    let datePacific = TinyMoon.formatDate(year: 2024, month: 02, day: 23, timeZone: pacificTimeZone)
    let dateUTC = TinyMoon.formatDate(year: 2024, month: 02, day: 23, timeZone: utcTimeZone)

    let moonPacific = TinyMoon.calculateMoonPhase(datePacific, timeZone: pacificTimeZone)
    let moonUTC = TinyMoon.calculateMoonPhase(dateUTC, timeZone: utcTimeZone)
    MoonTestHelper.prettyPrintMoonObject(moonPacific)
    MoonTestHelper.prettyPrintMoonObject(moonUTC)
  }

---

To debug:

• See if I get the same values from https://github.com/mourner/suncalc/tree/master and https://github.com/wlandsman/IDLAstro/blob/master/pro/mphase.pro
• If so, then it's not a bug in my code and I'll need to either find other calculations or
• If not, then find the bug in my code (best case scenario)

[mannylopez]

Notes

• Possible method to calculate the moonphase from two phase values: please provide round of function for phase. mourner/suncalc#114 (comment)

static func _getmoonPhase(phase1: Double, phase2: Double) -> Int {
      let percentages = [0, 0.25, 0.5, 0.75, 1]
      var index = 0

      if phase1 <= phase2 {
        for i in 0...percentages.count {
          if percentages[i] >= phase1, percentages[i] <= phase2 {
            index = 2 * i
            break
          } else if percentages[i] > phase1 {
            index = (2 * i) - 1
            break
          }
        }
      } else {
        index = 0
      }

      return index % 8
    }

and to test

let phases: [TinyMoon.MoonPhase] = [.newMoon, .waxingCrescent, .firstQuarter, .waxingGibbous, .fullMoon, .waningGibbous, .lastQuarter, .waningCrescent]

    let date1 = TinyMoon.formatDate(year: 2024, month: 7, day: 20, timeZone: utcTimeZone)
    let date2 = TinyMoon.formatDate(year: 2024, month: 07, day: 21, timeZone: utcTimeZone)
    let date3 = TinyMoon.formatDate(year: 2024, month: 07, day: 22, timeZone: utcTimeZone)

    let (start, end) = TinyMoon.Moon.julianStartAndEndOfDay(date: date2, timeZone: utcTimeZone)

    let p1 = TinyMoon.AstronomicalConstant.getMoonPhase(julianDay: start).phase
    let p2 = TinyMoon.AstronomicalConstant.getMoonPhase(julianDay: end).phase

    print("p1", p1)
    print("p2", p2)
    let index = TinyMoon.AstronomicalConstant._getmoonPhase(phase1: p1, phase2: p2)
    print(index)
    print(phases[index])
    ```

[mannylopez]

Ended up finding MoonTool and converting the source code from Visual C to Swift. Much more accurate.

[mannylopez]

Tests for these cases added here:

TinyMoon/Tests/TinyMoonTests/TinyMoonTests.swift

Lines 386 to 449 in 50b5dfd

	// These following moon phases happen within 5 hours of midnight, so these tests aim to check that the phase is calculated correctly and lands on the correct day
	func test_moon_marginOfError() {
	// 4 full moon (3 PST / 1 UTC)
	// 5 new moon (4 PST / 1 UTC)
	// 1 last quarter (1 UTC)
	// --------------
	// 10 total (7 PST / 3 UTC)
	// MARK: - PST margin of error tests
	// Values from https://www.timeanddate.com/moon/phases/usa/portland-or
	// Full moon on 2/24/2024 @ 04:30 PST
	var date = TinyMoon.formatDate(year: 2024, month: 2, day: 24, hour: 4, minute: 30, timeZone: pacificTimeZone)
	var moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .fullMoon)
	// New moon on 3/10/2024 @ 01:00 PST
	date = TinyMoon.formatDate(year: 2024, month: 3, day: 10, hour: 1, minute: 0, timeZone: pacificTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .newMoon)
	// Full moon on 3/25/2024 @ 00:00 PST
	date = TinyMoon.formatDate(year: 2024, month: 3, day: 25, hour: 0, minute: 0, timeZone: pacificTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .fullMoon)
	// New moon on 3/10/2024 @ 01:00 PST
	date = TinyMoon.formatDate(year: 2024, month: 3, day: 10, hour: 1, minute: 0, timeZone: pacificTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .newMoon)
	// New moon on 8/4/2024 @ 04:13 PST
	date = TinyMoon.formatDate(year: 2024, month: 8, day: 4, hour: 4, minute: 13, timeZone: pacificTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .newMoon)
	// New moon on 11/30/2024 @ 22:21 PST
	date = TinyMoon.formatDate(year: 2024, month: 11, day: 30, hour: 22, minute: 21, timeZone: pacificTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .newMoon)
	// Full moon on 12/15/2024 @ 01:01 PST
	date = TinyMoon.formatDate(year: 2024, month: 12, day: 15, hour: 1, minute: 01, timeZone: pacificTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: pacificTimeZone)
	XCTAssertEqual(moon.moonPhase, .fullMoon)
	// MARK: - UTC margin of error tests
	// Values from https://www.timeanddate.com/moon/phases/timezone/utc
	// Last Quarter moon on 4/2/2024 @ 03:14 UTC
	date = TinyMoon.formatDate(year: 2024, month: 4, day: 2, hour: 3, minute: 14, timeZone: utcTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: utcTimeZone)
	XCTAssertEqual(moon.moonPhase, .lastQuarter)
	// Full moon on 4/23/2024 @ 23:48 UTC
	date = TinyMoon.formatDate(year: 2024, month: 4, day: 23, hour: 23, minute: 48, timeZone: utcTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: utcTimeZone)
	XCTAssertEqual(moon.moonPhase, .fullMoon)
	// New moon on 9/3/2024 @ 01:55 UTC
	date = TinyMoon.formatDate(year: 2024, month: 9, day: 3, hour: 1, minute: 55, timeZone: utcTimeZone)
	moon = TinyMoon.calculateMoonPhase(date, timeZone: utcTimeZone)
	XCTAssertEqual(moon.moonPhase, .newMoon)
	}