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var PI = Math.PI;
var DR = PI/180;
var K1 = 15*DR*1.0027379

var Sunrise = false;
var Sunset  = 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];
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 sunrise_lit;
var sunset_lit;

Now = new Date();
srs_compute();


// compute ...
function srs_compute()
{
    var lat = 50.9518; // 50:57:06N
    var lon = -2.0449; // 02:02:42W

    sunrise_lit = "";
    sunset_lit  = "";
    srs_riseset(lat, lon);
}

// calculate sunrise and sunset times
function srs_riseset( lat, lon )
{
    var 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!");
    
    lon = lon/360;
    var tz  = zone/24;
    var ct  = jd/36525 + 1;                    // centuries since 1900.0
    var t0 = lst(lon, jd, tz);                 // local sidereal time

    jd = jd + tz;                              // get sun position at start of day
    sun(jd, ct);
    var ra0  = Sky[0];
    var dec0 = Sky[1];

    jd = jd + 1;                               // get sun position at end of day
    sun(jd, ct);
    var ra1  = Sky[0];
    var dec1 = Sky[1];

    if (ra1 < ra0)                             // make continuous 
        ra1 = ra1 + 2*PI;

    Sunrise = false;                           // initialize
    Sunset  = false;
    RAn[0]   = ra0;
    Dec[0]  = dec0;
    
    for (k = 0; k < 24; k++)                   // check each hour of this day
    {
        ph = (k + 1)/24;
        
        RAn[2] = ra0  + (k + 1)*(ra1  - ra0)/24;
        Dec[2] = dec0 + (k + 1)*(dec1 - dec0)/24;
        VHz[2] = test_hour(k, zone, t0, lat);
        
        RAn[0] = RAn[2];                       // advance to next hour
        Dec[0] = Dec[2];
        VHz[0] = VHz[2];
    }

    // display results
    sunrise_lit = zintstr(Rise_time[0], 2) + ":" + zintstr(Rise_time[1], 2) 
                       + ", az = " + frealstr(Rise_az, 5, 1) + "°";
    sunset_lit  = 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;
}

// test an hour for an event
function test_hour( 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;
        Sunrise = true;
    }
    
    if ((VHz[0] > 0)&&(VHz[2] < 0))
    {
        Set_time[0] = hr;
        Set_time[1] = min;
        Set_az = az;
        Sunset = true;
    }

    return VHz[2];
}

// check for no sunrise and/or no sunset
function special_message()
{
    if ((!Sunrise)&&(!Sunset))                 // neither sunrise nor sunset
    {
        if (VHz[2] < 0)
            sunrise_lit = "Sun down all day";
        else
            sunrise_lit = "Sun up all day";

        sunset_lit = "";
    }
    else                                       // sunrise or sunset
    {
        if (!Sunrise)
            sunrise_lit = "No sunrise this date";
        else if (!Sunset)
            sunset_lit  = "No sunset this date";
    }
}

// sun's position using fundamental arguments 
// (Van Flandern & Pulkkinen, 1979)
function sun( jd, ct )
{
    var g, lo, s, u, v, w;
    
    lo = 0.779072 + 0.00273790931*jd;
    lo = lo - Math.floor(lo);
    lo = lo*2*PI;

    g = 0.993126 + 0.0027377785*jd;
    g = g - Math.floor(g);
    g = g*2*PI;
    
    v = 0.39785*Math.sin(lo);
    v = v - 0.01*Math.sin(lo - g);
    v = v + 0.00333*Math.sin(lo + g);
    v = v - 0.00021*ct * Math.sin(lo);
    
    u = 1 - 0.03349*Math.cos(g);
    u = u - 0.00014*Math.cos(2*lo);
    u = u + 0.00008*Math.cos(lo);
    
    w = -0.0001 - 0.04129*Math.sin(2*lo);
    w = w + 0.03211*Math.sin(g );
    w = w + 0.00104*Math.sin(2*lo - g);
    w = w - 0.00035*Math.sin(2*lo + g);
    w = w - 0.00008*ct*Math.sin(g);
    
    s = w/Math.sqrt(u - v*v);                  // compute sun's right ascension  
    Sky[0] = lo + Math.atan(s/Math.sqrt(1 - s*s));

    s = v/Math.sqrt(u);                        // ...and declination 
    Sky[1] = Math.atan(s/Math.sqrt(1 - s*s));
}

// 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());
}

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