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Simplify Julian Day method #33

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Jul 10, 2024
Merged

Simplify Julian Day method #33

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49 changes: 9 additions & 40 deletions Sources/TinyMoon/TinyMoon+AstronomicalConstant.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -187,7 +187,7 @@ extension TinyMoon {

/// The number of Julian days since 1 January 2000, 12:00 UTC
///
/// `2451545.0` is the Julian date on 1 January 2000, 12:00 UTC, aka J2000.0
/// `2451545.0` is the Julian date on 1 January 2000, 12:00 UTC, aka J2000
static func daysSinceJ2000(from jd: Double) -> Double {
jd - 2451545.0
}
Expand All @@ -201,46 +201,15 @@ extension TinyMoon {
///
/// The Julian Day Count is a uniform count of days from a remote epoch in the past and is used for calculating the days between two events.
///
/// The Julian day is calculated by combining the contributions from the years, months, and day, taking into account constant offsets and rounding down the result.
/// https://quasar.as.utexas.edu/BillInfo/JulianDatesG.html
/// - Note: Uses hour, minute, and seconds to calculate a precise Julian Day
/// The Julian day is calculated by combining the contributions from the years, months, and day, taking into account constant
///
/// Formula based on https://github.com/mourner/suncalc/blob/master/suncalc.js#L29
/// and https://github.com/microsoft/AirSim/blob/main/AirLib/include/common/EarthCelestial.hpp#L115
/// - Note
/// - `2440588` is the Julian day for January 1, 1970, 12:00 UTC, aka J170
/// - `1000 * 60 * 60 * 24` is a day in milliseconds
static func julianDay(_ date: Date) -> Double {
var calendar = Calendar.current
calendar.timeZone = TimeZone(secondsFromGMT: 0) ?? calendar.timeZone
let components = calendar.dateComponents([.year, .month, .day, .hour, .minute, .second], from: date)

guard
let year = components.year,
let month = components.month,
let day = components.day,
let hour = components.hour,
let minute = components.minute,
let second = components.second
else {
fatalError("Could not extract date components")
}

/// Used to adjust January and February to be part of the previous year.
/// `14` is used to adjust the start of the year to March. Will either be `0` or `1`.
let a = (14 - month) / 12
/// The Julian Day calculation sets year 0 as 4713 BC.
/// To avoid working with negative numbers, `4800` is used as an offset.
let y = year + 4800 - a
/// Adjusts the month for the Julian Day calculation, where March is considered the first month of the year.
let m = month + 12 * a - 3

/// Calculate Julian Day Number for the date
/// - `153`: A magic number used for month length adjustments.
/// - `365`, `4`, `100`, and `400`: These relate to the number of days in a year and the correction for leap years in the Julian and Gregorian calendars.
/// `32045` is the correction factor to align the result with the Julian Day Number.
let jdn = Double(day) + Double((153 * m + 2) / 5) + Double(y) * 365 + Double(y / 4) - Double(y / 100) + Double(y / 400) -
32045

/// Calculate the fraction of the day past since midnight
/// `1440` is the number of minutes in a day, and `86400` is the number of seconds in a day
let dayFraction = (Double(hour) - 12) / 24 + Double(minute) / 1440 + Double(second) / 86400
let julianDayWithTime = jdn + dayFraction
return (julianDayWithTime * 10000) / 10000
(date.timeIntervalSince1970 * 1000) / (1000 * 60 * 60 * 24) - 0.5 + 2440588.0
}
}
}
34 changes: 29 additions & 5 deletions Tests/TinyMoonTests/AstronomicalConstantTests.swift
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Expand Up @@ -22,27 +22,51 @@ final class AstronomicalConstantTests: XCTestCase {
}

func test_astronomicalConstant_julianDay() {
// January 6, 2000 @ 00:00:00.0
let pacificTimeZone = TinyMoon.TimeZoneOption.createTimeZone(timeZone: .pacific)

// January 6, 2000 @ 00:00:00
var date = TinyMoon.formatDate(year: 2000, month: 01, day: 06)
var julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2451549.5000)

// January 6, 2000 @ 20:00:00.0
// January 6, 2000 @ 00:00: Pacific
// January 6, 2000 @ 08:00:00 UTC
date = TinyMoon.formatDate(
year: 2000,
month: 01,
day: 06,
hour: 0,
timeZone: pacificTimeZone)
julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2451549.8333333335)

// January 5, 2000 @ 16:00: Pacific
// January 6, 2000 @ 00:00:00 UTC
date = TinyMoon.formatDate(
year: 2000,
month: 01,
day: 5,
hour: 16,
timeZone: pacificTimeZone)
julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2451549.5000)

// January 6, 2000 @ 20:00:00
date = TinyMoon.formatDate(year: 2000, month: 01, day: 06, hour: 20, minute: 00)
julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2451550.3333333335)

// August 22, 2022 @ 00:00:00.0
// August 22, 2022 @ 00:00:00
date = TinyMoon.formatDate(year: 2022, month: 08, day: 22, hour: 00, minute: 00)
julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2459813.5000)

// August 22, 2022 @ 04:05:00.0
// August 22, 2022 @ 04:05:00
date = TinyMoon.formatDate(year: 2022, month: 08, day: 22, hour: 04, minute: 05)
julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2459813.670138889)

// August 22, 2022 @ 14:05:00.0
// August 22, 2022 @ 14:05:00
date = TinyMoon.formatDate(year: 2022, month: 08, day: 22, hour: 14, minute: 05)
julianDay = TinyMoon.AstronomicalConstant.julianDay(date)
XCTAssertEqual(julianDay, 2459814.0868055555)
Expand Down