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// Part 1: Converter functions
// Sin function with degree instead of radian
function sind(degree) {
return Math.sin(degree * Math.PI / 180)
}
// Convert Gregorian date to Julian day
function getJulianDay(date) {
const year = date.getFullYear()
const month = date.getMonth() + 1
const day = date.getDate()
let a = parseInt((14 - month) / 12)
let y = year + 4800 - a
y = y * 365 + parseInt(y / 4) - parseInt(y / 100) + parseInt(y / 400)
let m = month + 12 * a - 3
return parseInt(day + (153 * m + 2) / 5) + y - 32045
}
// Convert Julian day to Gregorian date
function getDateFromJulianDay(jd) {
const a = jd + 32044
const b = parseInt((4 * a + 3) / 146097)
const c = a - parseInt((b * 146097) / 4)
const d = parseInt((4 * c + 3) / 1461)
const e = c - parseInt((1461 * d) / 4)
const m = parseInt((5 * e + 2) / 153)
const dd = e - parseInt((153 * m + 2) / 5) + 1
const mm = m + 2 - 12 * parseInt(m / 10)
const yy = b * 100 + d - 4800 + parseInt(m / 10)
return new Date(yy, mm, dd)
}
// Get kth new moon day since 1900-01-01 in Julian day
function getNewMoonDay(k, tzOffset) {
const t = k / 1236.85
const t2 = t * t
const t3 = t2 * t
let jd1 = 2415020.75933 + 29.53058868 * k + 0.0001178 * t2 - 0.000000155 * t3
jd1 += 0.00033 * sind(166.56 + 132.87 * t - 0.009173 * t2)
const m = 359.2242 + 29.10535608 * k - 0.0000333 * t2 - 0.00000347 * t3
const mpr = 306.0253 + 385.81691806 * k + 0.0107306 * t2 + 0.00001236 * t3
const f = 21.2964 + 390.67050646 * k - 0.0016528 * t2 - 0.00000239 * t3
let c1 = (0.1734 - 0.000393 * t) * sind(m) + 0.0021 * sind(2 * m)
c1 -= 0.4068 * sind(mpr) + 0.0161 * sind(2 * mpr)
c1 -= 0.0004 * sind(3 * mpr)
c1 += 0.0104 * sind(2 * f) - 0.0051 * sind(m + mpr)
c1 -= 0.0074 * sind(m - mpr) + 0.0004 * sind(2 * f + m)
c1 -= 0.0004 * sind(2 * f - m) - 0.0006 * sind(2 * f + mpr)
c1 += 0.0010 * sind(2 * f - mpr) + 0.0005 * sind(2 * mpr + m)
let dt
if (t < -11) {
dt = 0.001 + 0.000839 * t + 0.0002261 * t2 - 0.00000845 * t3 - 0.000000081 * t * t3
} else {
dt = -0.000278 + 0.000265 * t + 0.000262 * t2
}
let jdNew = jd1 + c1 - dt
return parseInt(jdNew + 0.5 + tzOffset / 24)
}
// Get solar term from Julian day, from 0 to 23, corresponding to Chunfen to Jingzhe.
function getSolarTerm(jd, tzOffset) {
t = (jd - 2451545.5 - tzOffset / 24) / 36525
t2 = t * t
m = 357.52910 + 35999.05030 * t - 0.0001559 * t2 - 0.00000048 * t * t2
l0 = 280.46645 + 36000.76983 * t + 0.0003032 * t2
dl = (1.914600 - 0.004817 * t - 0.000014 * t2) * sind(m)
dl = dl + (0.019993 - 0.000101 * t) * sind(2 * m) + 0.000290 * sind(3 * m)
l = l0 + dl
l %= 360 // normalize to (0, 360)
return parseInt(l * 24 / 360)
}
// Get the solar longitude of the day, from 0 to 11.
// 0 is corresponding to 0 degree to 30 degree and solar term Chunfen (spring equinox) - Guyu
// 1 is corresponding to 30 degree to 60 degree and solar term Guyu - Xiaoman
// 2 is corresponding to 60 degree to 90 degree and solar term Xiaoman - Xiazhi (summer solstince)
// etc
function getSunLongitude(jd, tzOffset) {
t = (jd - 2451545.5 - tzOffset / 24) / 36525
t2 = t * t
m = 357.52910 + 35999.05030 * t - 0.0001559 * t2 - 0.00000048 * t * t2
l0 = 280.46645 + 36000.76983 * t + 0.0003032 * t2
dl = (1.914600 - 0.004817 * t - 0.000014 * t2) * sind(m)
dl = dl + (0.019993 - 0.000101 * t) * sind(2 * m) + 0.000290 * sind(3 * m)
l = l0 + dl
l %= 360 // normalize to (0, 360 deg)
return parseInt(l * 12 / 360)
}
// Get Julian day for the first day of 11th lunar month of the year.
function getMonth11(year, tzOffset) {
off = getJulianDay(new Date(year, 11, 31)) - 2415021
k = parseInt(off / 29.530588853)
nm = getNewMoonDay(k, tzOffset)
sun_long = getSunLongitude(nm, tzOffset)
if (sun_long >= 9) {
nm = getNewMoonDay(k-1, tzOffset)
}
return nm
}
// Get the leap month offset from the 11th month before it.
//
// a11: Julian day of the first day of the 11th month
function getLeapMonthOffset(a11, tzOffset) {
k = parseInt((a11 - 2415021.076998695) / 29.530588853 + 0.5)
i = 1
arc = getSunLongitude(getNewMoonDay(k + i, tzOffset), tzOffset)
last = 0
while (arc != last && i < 14) {
last = arc
i++
arc = getSunLongitude(getNewMoonDay(k + i, tzOffset), tzOffset)
}
return i - 1
}
function convertSolarToLunar(date, tzOffset) {
const jd = getJulianDay(date)
let k = parseInt((jd - 2415021.076998695) / 29.530588853)
let monthStart = getNewMoonDay(k + 1, tzOffset)
if (monthStart > jd) {
monthStart = getNewMoonDay(k, tzOffset)
}
let a11 = getMonth11(date.getFullYear(), tzOffset)
let b11 = a11
let lunarYear
if (a11 >= monthStart) {
lunarYear = date.getFullYear()
a11 = getMonth11(lunarYear - 1, tzOffset)
} else {
lunarYear = date.getFullYear() + 1
b11 = getMonth11(lunarYear, tzOffset)
}
let lunarDay = jd - monthStart + 1
let diff = parseInt((monthStart - a11) / 29)
let isLeapMonth = false
let lunarMonth = diff + 11
if (b11 - a11 > 365) {
leapMonthDiff = getLeapMonthOffset(a11, tzOffset)
if (diff >= leapMonthDiff) {
lunarMonth = diff + 10
if (diff == leapMonthDiff) {
isLeapMonth = true
}
}
}
if (lunarMonth > 12) {
lunarMonth -= 12
}
if (lunarMonth >= 11 && diff < 4) {
lunarYear -= 1
}
return {
year: lunarYear,
month: lunarMonth,
day: lunarDay,
isLeap: isLeapMonth
}
}
// Part 2: Update DOM
// DOM
const dateInput = document.querySelector('#solar-date')
const tzOffsetInput = document.querySelector('#timezone-offset')
const solarYearOutput = document.querySelector('#solar-year')
const solarMonthOutput = document.querySelector('#solar-month')
const solarDayOutput = document.querySelector('#solar-day')
const solarWeekdayOutput = document.querySelector('#solar-weekday')
const lunarYearOutput = document.querySelector('#lunar-year')
const lunarMonthOutput = document.querySelector('#lunar-month')
const lunarDayOutput = document.querySelector('#lunar-day')
const monthFormat = new Intl.DateTimeFormat("en-US", {month: "long"})
const weekdayFormat = new Intl.DateTimeFormat("en-US", {weekday: "long"})
function updateOutputs() {
if (dateInput.valueAsDate === null) {
dateInput.valueAsDate = new Date()
}
const date = dateInput.valueAsDate
const tzOffset = parseInt(tzOffsetInput.value)
solarYearOutput.innerText = date.getFullYear()
solarMonthOutput.innerText = monthFormat.format(date)
solarDayOutput.innerText = date.getDate()
solarWeekdayOutput.innerText = weekdayFormat.format(date)
const lunarDate = convertSolarToLunar(date, tzOffset)
lunarYearOutput.innerText = lunarDate.year
lunarMonthOutput.innerText = lunarDate.month
if (lunarDate.isLeap) {
lunarMonthOutput.innerText += " (leap)"
}
lunarDayOutput.innerText = lunarDate.day
}
document.addEventListener("DOMContentLoaded", updateOutputs)
dateInput.onchange = updateOutputs
tzOffsetInput.onchange = updateOutputs
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