問題

你想找出月球的相位。

解決方案

以下代碼提供了一種計算給出日期的月球相位計算方案：

# moonPhase.coffee

# Moon-phase calculator
# Roger W. Sinnott, Sky & Telescope, June 16, 2006
# http://www.skyandtelescope.com/observing/objects/javascript/moon_phases
#
# Translated to CoffeeScript by Mike Hatfield @WebCoding4Fun

proper_ang = (big) ->
    tmp = 0
    if big > 0
        tmp = big / 360.0
        tmp = (tmp - (~~tmp)) * 360.0
    else
        tmp = Math.ceil(Math.abs(big / 360.0))
        tmp = big + tmp * 360.0

    tmp

jdn = (date) ->  
    month = date.getMonth()
    day = date.getDate()
    year = date.getFullYear()
    zone = date.getTimezoneOffset() / 1440

    mm = month
    dd = day
    yy = year

    yyy = yy
    mmm = mm
    if mm < 3
        yyy = yyy - 1
        mmm = mm + 12

    day = dd + zone + 0.5
    a = ~~( yyy / 100 )
    b = 2 - a + ~~( a / 4 )
    jd = ~~( 365.25 * yyy ) + ~~( 30.6001 * ( mmm+ 1 ) ) + day + 1720994.5
    jd + b if jd > 2299160.4999999

moonElong = (jd) ->
    dr    = Math.PI / 180
    rd    = 1 / dr
    meeDT = Math.pow((jd - 2382148), 2) / (41048480 * 86400)
    meeT  = (jd + meeDT - 2451545.0) / 36525
    meeT2 = Math.pow(meeT, 2)
    meeT3 = Math.pow(meeT, 3)
    meeD  = 297.85 + (445267.1115 * meeT) - (0.0016300 * meeT2) + (meeT3 / 545868)
    meeD  = (proper_ang meeD) * dr
    meeM1 = 134.96 + (477198.8676 * meeT) + (0.0089970 * meeT2) + (meeT3 / 69699)
    meeM1 = (proper_ang meeM1) * dr
    meeM  = 357.53 + (35999.0503 * meeT)
    meeM  = (proper_ang meeM) * dr

    elong = meeD * rd + 6.29 * Math.sin( meeM1 )
    elong = elong     - 2.10 * Math.sin( meeM )
    elong = elong     + 1.27 * Math.sin( 2*meeD - meeM1 )
    elong = elong     + 0.66 * Math.sin( 2*meeD )
    elong = proper_ang elong
    elong = Math.round elong

    moonNum = ( ( elong + 6.43 ) / 360 ) * 28
    moonNum = ~~( moonNum )

    if moonNum is 28 then 0 else moonNum

getMoonPhase = (age) ->
    moonPhase = "new Moon"
    moonPhase = "first quarter" if age > 3 and age < 11 
    moonPhase = "full Moon"     if age > 10 and age < 18
    moonPhase = "last quarter"  if age > 17 and age < 25

    if ((age is 1) or (age is 8) or (age is 15) or (age is 22))
        moonPhase = "1 day past " + moonPhase

    if ((age is 2) or (age is 9) or (age is 16) or (age is 23))
        moonPhase = "2 days past " + moonPhase

    if ((age is 3) or (age is 1) or (age is 17) or (age is 24))
        moonPhase = "3 days past " + moonPhase

    if ((age is 4) or (age is 11) or (age is 18) or (age is 25))
        moonPhase = "3 days before " + moonPhase

    if ((age is 5) or (age is 12) or (age is 19) or (age is 26))
        moonPhase = "2 days before " + moonPhase

    if ((age is 6) or (age is 13) or (age is 20) or (age is 27))
        moonPhase = "1 day before " + moonPhase

    moonPhase

MoonPhase = exports? and exports or @MoonPhase = {}

class MoonPhase.Calculator
    getMoonDays: (date) ->
        jd = jdn date 
        moonElong jd

    getMoonPhase: (date) ->      
        jd = jdn date 
        getMoonPhase( moonElong jd )

討論

此代碼顯示一個月球相位計算器對象的方法有兩種。計算器 -> getmoonphase 將返回一用個文本表示的日期的月球相位。

這可以用在瀏覽器和 Node.js 中。

$ node
> var MoonPhase = require('./moonPhase.js');
 undefined
> var calc = new MoonPhase.Calculator();
 undefined
> calc.getMoonPhase(new Date());
 'full moon'
> calc.getMoonPhase(new Date(1972, 6, 30));
 '3 days before last quarter'