# CoffeeScript 计算月球的相位

## 解决方案

``````# 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 )``````

## 讨论

``````\$ 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'``````

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