// JavaScript Document

<!-- hide
var PI = Math.PI;
var DR = PI/180;
var K1 = 15*DR*1.0027379
var Moonrise = false;
var Moonset  = false;
var Rise_time = [0, 0];
var Set_time  = [0, 0];
var Rise_az = 0.0;
var Set_az  = 0.0;
var Sky = [0.0, 0.0, 0.0];
var RAn = [0.0, 0.0, 0.0];
var Dec = [0.0, 0.0, 0.0];
var VHz = [0.0, 0.0, 0.0];
var Now;

var lat_degrees = "42";
var lat_minutes = "21";
var lon_degrees = "71";
var lon_minutes = "04";

// initialize date
function initdate()
{
    Now = new Date();
    showdate(Now);
}

// display the date
function showdate(d)
{
    var winter = new Date(2001, 11, 30);       // for northern hemisphere
    var summer = new Date(2001,  5, 30);
    var summer_tz = summer.getTimezoneOffset();
    var winter_tz = winter.getTimezoneOffset();
    var Local_dst = (summer_tz == winter_tz) ? false : true;

    if (summer_tz > winter_tz)                 // in southern hemisphere
    {
        var tz = summer_tz;                    // swap
        summer_tz = winter_tz;
        winter_tz = tz;
    }

    var thisday = (d.getMonth() + 1) + "/";
    thisday    += d.getDate() + "/";
    thisday    += d.getFullYear();

    if ((Local_dst == true)&&(summer_tz == d.getTimezoneOffset()))
        thisday += " [DST]";

	document.getElementById( 'thisday' ).innerHTML = thisday;
}

// compute ...
function compute()
{
    var latd = parseInt(lat_degrees, 10);
    var latm = parseInt(lat_minutes, 10);
    var lond = parseInt(lon_degrees, 10);
    var lonm = parseInt(lon_minutes, 10);

    var lat = latd + latm/60.0;
    var lon = lond + lonm/60.0;

    lon = -lon;
    showdate(Now);
    riseset(lat, lon);
}

// do next day
function advance()
{
    var date = Now.getDate();
    Now.setDate(date + 1);
    compute();
}

// do previous day
function backup()
{
    var date = Now.getDate();
    Now.setDate(date - 1);
    compute();
}

// calculate moonrise and moonset times
function riseset( lat, lon )
{
    var i, j, k;
    var zone = Math.round(Now.getTimezoneOffset()/60);
    var jd = julian_day() - 2451545;           // Julian day relative to Jan 1.5, 2000

    if ((sgn(zone) == sgn(lon))&&(zone != 0))
        window.alert("WARNING: time zone and longitude are incompatible!");
    var mp = new Array(3);                     // create a 3x3 array
    for (i = 0; i < 3; i++)
    {
        mp[i] = new Array(3);
        for (j = 0; j < 3; j++)
            mp[i][j] = 0.0;
    }

    lon = lon/360;
    var tz = zone/24;
    var t0 = lst(lon, jd, tz);                 // local sidereal time

    jd = jd + tz;                              // get moon position at start of day

    for (k = 0; k < 3; k++)
    {
        moon(jd);
        mp[k][0] = Sky[0];
        mp[k][1] = Sky[1];
        mp[k][2] = Sky[2];
        jd = jd + 0.5;
    }

    if (mp[1][0] <= mp[0][0])
        mp[1][0] = mp[1][0] + 2*PI;

    if (mp[2][0] <= mp[1][0])
        mp[2][0] = mp[2][0] + 2*PI;

    RAn[0] = mp[0][0];
    Dec[0] = mp[0][1];

    Moonrise = false;                          // initialize
    Moonset  = false;

    for (k = 0; k < 24; k++)                   // check each hour of this day
    {
        ph = (k + 1)/24;

        RAn[2] = interpolate(mp[0][0], mp[1][0], mp[2][0], ph);
        Dec[2] = interpolate(mp[0][1], mp[1][1], mp[2][1], ph);

        VHz[2] = test_moon(k, zone, t0, lat, mp[1][2]);
        RAn[0] = RAn[2];                       // advance to next hour
        Dec[0] = Dec[2];
        VHz[0] = VHz[2];
    }

    // display results
    document.getElementById('moonrise').innerHTML = zintstr(Rise_time[0], 2) + ":" + zintstr(Rise_time[1], 2)
                       + ", az = " + frealstr(Rise_az, 5, 1) + "°";

    document.getElementById('moonset').innerHTML  = zintstr( Set_time[0], 2) + ":" + zintstr( Set_time[1], 2)
                       + ", az = " + frealstr(Set_az, 5, 1) + "°";

    special_message();
}

// Local Sidereal Time for zone
function lst( lon, jd, z )
{
    var s = 24110.5 + 8640184.812999999*jd/36525 + 86636.6*z + 86400*lon;
    s = s/86400;
    s = s - Math.floor(s);
    return s*360*DR;
}

// 3-point interpolation
function interpolate( f0, f1, f2, p )
{
    var a = f1 - f0;
    var b = f2 - f1 - a;
    var f = f0 + p*(2*a + b*(2*p - 1));

    return f;
}

// test an hour for an event
function test_moon( k, zone, t0, lat, plx )
{
    var ha = [0.0, 0.0, 0.0];
    var a, b, c, d, e, s, z;
    var hr, min, time;
    var az, hz, nz, dz;

    if (RAn[2] < RAn[0])
        RAn[2] = RAn[2] + 2*PI;

    ha[0] = t0 - RAn[0] + k*K1;
    ha[2] = t0 - RAn[2] + k*K1 + K1;

    ha[1]  = (ha[2] + ha[0])/2;                // hour angle at half hour
    Dec[1] = (Dec[2] + Dec[0])/2;              // declination at half hour

    s = Math.sin(DR*lat);
    c = Math.cos(DR*lat);

    // refraction + sun semidiameter at horizon + parallax correction
    z = Math.cos(DR*(90.567 - 41.685/plx));

    if (k <= 0)                                // first call of function
        VHz[0] = s*Math.sin(Dec[0]) + c*Math.cos(Dec[0])*Math.cos(ha[0]) - z;

    VHz[2] = s*Math.sin(Dec[2]) + c*Math.cos(Dec[2])*Math.cos(ha[2]) - z;

    if (sgn(VHz[0]) == sgn(VHz[2]))
        return VHz[2];                         // no event this hour

    VHz[1] = s*Math.sin(Dec[1]) + c*Math.cos(Dec[1])*Math.cos(ha[1]) - z;

    a = 2*VHz[2] - 4*VHz[1] + 2*VHz[0];
    b = 4*VHz[1] - 3*VHz[0] - VHz[2];
    d = b*b - 4*a*VHz[0];

    if (d < 0)
        return VHz[2];                         // no event this hour

    d = Math.sqrt(d);
    e = (-b + d)/(2*a);

    if (( e > 1 )||( e < 0 ))
        e = (-b - d)/(2*a);

    time = k + e + 1/120;                      // time of an event + round up
    hr   = Math.floor(time);
    min  = Math.floor((time - hr)*60);

    hz = ha[0] + e*(ha[2] - ha[0]);            // azimuth of the moon at the event
    nz = -Math.cos(Dec[1])*Math.sin(hz);
    dz = c*Math.sin(Dec[1]) - s*Math.cos(Dec[1])*Math.cos(hz);
    az = Math.atan2(nz, dz)/DR;
    if (az < 0) az = az + 360;

    if ((VHz[0] < 0)&&(VHz[2] > 0))
    {
        Rise_time[0] = hr;
        Rise_time[1] = min;
        Rise_az = az;
        Moonrise = true;
    }

    if ((VHz[0] > 0)&&(VHz[2] < 0))
    {
        Set_time[0] = hr;
        Set_time[1] = min;
        Set_az = az;
        Moonset = true;
    }

    return VHz[2];
}

// test an hour for an event
function test_sun( k, zone, t0, lat )
{
    var ha = new Array(3);
    var a, b, c, d, e, s, z;
    var hr, min, time;
    var az, dz, hz, nz;

    ha[0] = t0 - RAn[0] + k*K1;
    ha[2] = t0 - RAn[2] + k*K1 + K1;

    ha[1]  = (ha[2]  + ha[0])/2;               // hour angle at half hour
    Dec[1] = (Dec[2] + Dec[0])/2 ;             // declination at half hour

    s = Math.sin(lat*DR);
    c = Math.cos(lat*DR);
    z = Math.cos(90.833*DR);                   // refraction + sun semidiameter at horizon

    if (k <= 0)
        VHz[0] = s*Math.sin(Dec[0]) + c*Math.cos(Dec[0])*Math.cos(ha[0]) - z;

    VHz[2] = s*Math.sin(Dec[2]) + c*Math.cos(Dec[2])*Math.cos(ha[2]) - z;

    if (sgn(VHz[0]) == sgn(VHz[2]))
        return VHz[2];                         // no event this hour

    VHz[1] = s*Math.sin(Dec[1]) + c*Math.cos(Dec[1])*Math.cos(ha[1]) - z;

    a =  2* VHz[0] - 4*VHz[1] + 2*VHz[2];
    b = -3* VHz[0] + 4*VHz[1] - VHz[2];
    d = b*b - 4*a*VHz[0];

    if (d < 0)
        return VHz[2];                         // no event this hour

    d = Math.sqrt(d);
    e = (-b + d)/(2 * a);

    if ((e > 1)||(e < 0))
        e = (-b - d)/(2*a);

    time = k + e + 1/120;                      // time of an event

    hr = Math.floor(time);
    min = Math.floor((time - hr)*60);

    hz = ha[0] + e*(ha[2] - ha[0]);            // azimuth of the sun at the event
    nz = -Math.cos(Dec[1])*Math.sin(hz);
    dz = c*Math.sin(Dec[1]) - s*Math.cos(Dec[1])*Math.cos(hz);
    az = Math.atan2(nz, dz)/DR;

    if (az < 0) az = az + 360;

    if ((VHz[0] < 0)&&(VHz[2] > 0))
    {
        Rise_time[0] = hr;
        Rise_time[1] = min;
        Rise_az = az;
        Moonrise = true;
    }

    if ((VHz[0] > 0)&&(VHz[2] < 0))
    {
        Set_time[0] = hr;
        Set_time[1] = min;
        Set_az = az;
        Moonset = true;
    }

    return VHz[2];
}

// check for no moonrise and/or no moonset
function special_message()
{
    if ((!Moonrise)&&(!Moonset))               // neither moonrise nor moonset
    {
        if (VHz[2] < 0)
            document.getElementById('moonrise').innerHTML = "Moon down all day";
        else
            document.getElementById('moonrise').innerHTML = "Moon up all day";

        document.getElementById('moonset').innerHTML = "";
    }
    else                                       // moonrise or moonset
    {
        if (!Moonrise)
            document.getElementById('moonrise').innerHTML = "No moonrise this date";
        else if (!Moonset)
            document.getElementById('moonset').innerHTML  = "No moonset this date";
    }
}

// moon's position using fundamental arguments
// (Van Flandern & Pulkkinen, 1979)
function moon( jd )
{
    var d, f, g, h, m, n, s, u, v, w;

    h = 0.606434 + 0.03660110129*jd;
    m = 0.374897 + 0.03629164709*jd;
    f = 0.259091 + 0.0367481952 *jd;
    d = 0.827362 + 0.03386319198*jd;
    n = 0.347343 - 0.00014709391*jd;
    g = 0.993126 + 0.0027377785 *jd;

    h = h - Math.floor(h);
    m = m - Math.floor(m);
    f = f - Math.floor(f);
    d = d - Math.floor(d);
    n = n - Math.floor(n);
    g = g - Math.floor(g);


    h = h*2*PI;
    m = m*2*PI;
    f = f*2*PI;
    d = d*2*PI;
    n = n*2*PI;
    g = g*2*PI;

    v = 0.39558*Math.sin(f + n);
    v = v + 0.082  *Math.sin(f);
    v = v + 0.03257*Math.sin(m - f - n);
    v = v + 0.01092*Math.sin(m + f + n);
    v = v + 0.00666*Math.sin(m - f);
    v = v - 0.00644*Math.sin(m + f - 2*d + n);
    v = v - 0.00331*Math.sin(f - 2*d + n);
    v = v - 0.00304*Math.sin(f - 2*d);
    v = v - 0.0024 *Math.sin(m - f - 2*d - n);
    v = v + 0.00226*Math.sin(m + f);
    v = v - 0.00108*Math.sin(m + f - 2*d);
    v = v - 0.00079*Math.sin(f - n);
    v = v + 0.00078*Math.sin(f + 2*d + n);

    u = 1 - 0.10828*Math.cos(m);
    u = u - 0.0188 *Math.cos(m - 2*d);
    u = u - 0.01479*Math.cos(2*d);
    u = u + 0.00181*Math.cos(2*m - 2*d);
    u = u - 0.00147*Math.cos(2*m);
    u = u - 0.00105*Math.cos(2*d - g);
    u = u - 0.00075*Math.cos(m - 2*d + g);

    w = 0.10478*Math.sin(m);
    w = w - 0.04105*Math.sin(2*f + 2*n);
    w = w - 0.0213 *Math.sin(m - 2*d);
    w = w - 0.01779*Math.sin(2*f + n);
    w = w + 0.01774*Math.sin(n);
    w = w + 0.00987*Math.sin(2*d);
    w = w - 0.00338*Math.sin(m - 2*f - 2*n);
    w = w - 0.00309*Math.sin(g);
    w = w - 0.0019 *Math.sin(2*f);
    w = w - 0.00144*Math.sin(m + n);
    w = w - 0.00144*Math.sin(m - 2*f - n);
    w = w - 0.00113*Math.sin(m + 2*f + 2*n);
    w = w - 0.00094*Math.sin(m - 2*d + g);
    w = w - 0.00092*Math.sin(2*m - 2*d);

    s = w/Math.sqrt(u - v*v);                  // compute moon's right ascension ...
    Sky[0] = h + Math.atan(s/Math.sqrt(1 - s*s));

    s = v/Math.sqrt(u);                        // declination ...
    Sky[1] = Math.atan(s/Math.sqrt(1 - s*s));
    Sky[2] = 60.40974*Math.sqrt( u );          // and parallax
}

// determine Julian day from calendar date
// (Jean Meeus, "Astronomical Algorithms", Willmann-Bell, 1991)
function julian_day()
{
    var a, b, jd;
    var gregorian;

    var month = Now.getMonth() + 1;
    var day   = Now.getDate();
    var year  = Now.getFullYear();

    gregorian = (year < 1583) ? false : true;

    if ((month == 1)||(month == 2))
    {
        year  = year  - 1;
        month = month + 12;
    }

    a = Math.floor(year/100);
    if (gregorian) b = 2 - a + Math.floor(a/4);
    else           b = 0.0;

    jd = Math.floor(365.25*(year + 4716))
       + Math.floor(30.6001*(month + 1))
       + day + b - 1524.5;

    return jd;
}

// returns value for sign of argument
function sgn( x )
{
    var rv;
    if (x > 0.0)      rv =  1;
    else if (x < 0.0) rv = -1;
    else              rv =  0;
    return rv;
}

// format a positive integer with leading zeroes
function zintstr( num, width )
{
    var str = num.toString(10);
    var len = str.length;
    var intgr = "";
    var i;

    for (i = 0; i < width - len; i++)          // append leading zeroes
        intgr += '0';

    for (i = 0; i < len; i++)                  // append digits
        intgr += str.charAt(i);

    return intgr;
}

// format an integer
function cintstr( num, width )
{
    var str = num.toString(10);
    var len = str.length;
    var intgr = "";
    var i;

    for (i = 0; i < width - len; i++)          // append leading spaces
        intgr += ' ';

    for (i = 0; i < len; i++)                  // append digits
        intgr += str.charAt(i);

   return intgr;
}

// format a real number
function frealstr( num, width, fract )
{
    var str = num.toFixed(fract);
    var len = str.length;
    var real = "";
    var i;

    for (i = 0; i < width - len; i++)          // append leading spaces
        real += ' ';

    for (i = 0; i < len; i++)                  // append digits
        real += str.charAt(i);

    return real;
}

// read data from cookie
function getCookie( name )
{
    if (document.cookie.length > 0)
    {
        begin = document.cookie.indexOf(name+"=");
        if (begin != -1) // Note: != means "is not equal to"
        {
            begin += name.length + 1;
            end = document.cookie.indexOf(";", begin);
            if (end == -1) end = document.cookie.length;

            return unescape(document.cookie.substring(begin, end));
        }
    }
    return null;
}

// write data to cookie
function setCookie( name, value, expiredays )
{
    var ExpireDate = new Date();
    ExpireDate.setTime(ExpireDate.getTime() + (expiredays*24*3600*1000));

    document.cookie = name
                    + "=" + escape(value)
                    + ((expiredays == null) ? "" : "; expires=" + ExpireDate.toGMTString());
}
// unhide -->