Moon phase calculation

moonobject.js

var MoonInfo = function(day, month, year) {

    var n0 = parseInt( "0" );
    var f0 = parseFloat( "0.0" );
    var AG = f0;   // Moon's age
    var DI = f0;   // Moon's distance in earth radii
    var LA = f0;   // Moon's ecliptic latitude
    var LO = f0;   // Moon's ecliptic longitude
    var Phase = " ";
    var Zodiac = " ";

    // Related to month length and age calculations
    var n28 = parseInt( "28" );
    var n30 = parseInt( "30" );
    var n31 = parseInt( "31" );
    var dim = new Array( n31, n28, n31, n30, n31, n30, n31, n31, n30, n31, n30, n31 );

    day = parseInt( day, 10 );
    month = parseInt( month, 10 );
    year = parseInt( year, 10 );

    moon_posit( year, month, day );

    this.age = round2( AG );
    this.distance = round2( DI );
    this.phase = Phase;
    this.latitude = round2( LA );
    this.longitude = round2( LO );
    this.zodiac = Zodiac;

    function isdayofmonth( y, m, d ) {
        if( m != 2 ) {
            if( 1 <= d && d <= dim[m-1] ) {
                return true;
            } else {
                return false;
            };
        }

        var feb = dim[1];
        if( isleapyear( y ) ) feb += 1;                                   // is leap year
        if( 1 <= d && d <= feb ) return true;
        return false;
    }

    function isleapyear( y ) {
        var x = Math.floor( y - 4*Math.floor( y/4 ) );
        var w = Math.floor( y - 100*Math.floor( y/100 ) );
        var z = Math.floor( y - 400*Math.floor( y/400 ) );

        if( x == 0 ) {                           // possible leap year
            if( w == 0 && z != 0 ) {
                return false;                  // not leap year
            } else {
                return true;                   // is leap year
            };
        }

        return false;
    }

    // compute moon position and phase
    function moon_posit( Y, M, D ) {
        var YY = n0,
            MM = n0,
            K1 = n0, 
            K2 = n0, 
            K3 = n0,
            JD = n0,
            IP = f0,
            DP = f0,
            NP = f0,
            RP = f0;
        
        // calculate the Julian date at 12h UT
        YY = Y - Math.floor( ( 12 - M ) / 10 );       
        MM = M + 9; 
        if( MM >= 12 ) MM = MM - 12;
        
        K1 = Math.floor( 365.25 * ( YY + 4712 ) );
        K2 = Math.floor( 30.6 * MM + 0.5 );
        K3 = Math.floor( Math.floor( ( YY / 100 ) + 49 ) * 0.75 ) - 38;
        
        JD = K1 + K2 + D + 59;                  // for dates in Julian calendar
        if( JD > 2299160 ) JD = JD - K3;        // for Gregorian calendar
            
        // calculate moon's age in days
        IP = normalize( ( JD - 2451550.1 ) / 29.530588853 );
        AG = IP*29.53;
        
        if(      AG <  1.84566 ) Phase = "A new moon";
        else if( AG <  5.53699 ) Phase = "An evening crescent";
        else if( AG <  9.22831 ) Phase = "A first quarter";
        else if( AG < 12.91963 ) Phase = "A waxing gibbous";
        else if( AG < 16.61096 ) Phase = "A full moon";
        else if( AG < 20.30228 ) Phase = "A waning gibbous";
        else if( AG < 23.99361 ) Phase = "A Last quarter";
        else if( AG < 27.68493 ) Phase = "A Morning crescent";
        else                     Phase = "A new moon";

        IP = IP*2*Math.PI;                      // Convert phase to radians

        // calculate moon's distance
        DP = 2*Math.PI*normalize( ( JD - 2451562.2 ) / 27.55454988 );
        DI = 60.4 - 3.3*Math.cos( DP ) - 0.6*Math.cos( 2*IP - DP ) - 0.5*Math.cos( 2*IP );

        // calculate moon's ecliptic latitude
        NP = 2*Math.PI*normalize( ( JD - 2451565.2 ) / 27.212220817 );
        LA = 5.1*Math.sin( NP );

        // calculate moon's ecliptic longitude
        RP = normalize( ( JD - 2451555.8 ) / 27.321582241 );
        LO = 360*RP + 6.3*Math.sin( DP ) + 1.3*Math.sin( 2*IP - DP ) + 0.7*Math.sin( 2*IP );

        // correcting if longitude is not greater than 360!
        if ( LO > 360 ) LO = LO - 360;
    }

    // round to 2 decimal places
    function round2( x )
    {
        return ( Math.round( 100*x )/100.0 );
    }
        
    // normalize values to range 0...1    
    function normalize( v )
    {
        v = v - Math.floor( v  ); 
        if( v < 0 )
            v = v + 1;
            
        return v;
    }

    if( !isdayofmonth( year, month, day ) ) {
        alert( "Invalid date" );
        return;
    }

};