<%@LANGUAGE="JSCRIPT"%><%
//
//   Display sunset and sunrise times for the current evening current place
//
// 22-Aug-11  rab Adapted from .vbs script
//	          Created to run in a tiddler instead of in the console
//		      Tweaked the SysJulianDate slightly (see below)
// 23-Sep-11  rbd Part of standard ACP UI. Credit Berg!
// 21-Sep-12  rbd GEM:871 Incorporate Robert Wahlström's high latitude changes
// 02-Feb-13  esy updated to calculate the dusk and dawn for the
//                current Astronomical day. If past dawn it will
//                calculate the next instead (same as if it was past
//                local noon). Also, changed the calculator to be a
//                function, rather than a "printer".
// 02-Feb-13  rab limited  arccos test to be [-1 to +1] .
//                regression tested for mid-Winter high latitudes --> OK

var RADIANS_MULTIPLE = 0.0174532925              // multiply by degrees to get radians

var        SUNSET = -0.833333 * RADIANS_MULTIPLE; // upper limb at horizon, includes refraction correction
var         CIVIL =      -6.0 * RADIANS_MULTIPLE; // need artificial illumination to read outside
var      NAUTICAL =     -12.0 * RADIANS_MULTIPLE; // navigation using sea horizon no longer possible
var       AMATEUR =     -15.0 * RADIANS_MULTIPLE; // dark enough for most astronomical observations
var  ASTRONOMICAL =     -18.0 * RADIANS_MULTIPLE; // sky is completely dark

var   JULIAN_HOUR = 0.0417			  // 1 hour as a fraction of a Julian Day
var JULIAN_SECOND = 0.0000115741		  // 1 second as a fraction of a Julian Day

var DAWN = true ; var DUSK = false;		  // set some vars to make code cleaner for dawn or dusk
var TODAY = false ; var TOMORROW = true;

function this_twilight(theta_S, dawn_or_dusk, next_day) {
    // takes the solar elevation angle in radians and returns the
    // twilight ends (dusk) or starts (dawn) during this "astronomical day".

    var KT = new ActiveXObject("Kepler.Ephemeris"); // Re-usable Earth ephem (reverse = Sun!)
    KT.BodyType = 0; KT.Number = 3;	            // This is a Planet. Earth.

    var tvec = new ActiveXObject("NOVAS.PositionVector");	    
    var latitude = Telescope.SiteLatitude * RADIANS_MULTIPLE; 

    var JD = Util.SysJulianDate;                    // Current UT; Start with tomorrow's date/Time
    var LOCAL_JD = JD - (Util.SysUTCOffset * JULIAN_HOUR) ;
    var LOCAL_JDN = Math.floor(LOCAL_JD);
    
    // Are we looking for a dawn or dsuk calculation?
    if (dawn_or_dusk)
	JD = LOCAL_JDN + 1;
    else
	JD = LOCAL_JDN;

    // similarly, this is here to do the next astronomical day if we
    // are already past dawn.
    if (next_day)
	JD = JD + 1;
    else
	JD = JD;

    var JDPrev = LOCAL_JDN;

    do {						  // Iterate through until we converge on a solution.
      	var KA = KT.GetPositionAndVelocity(JD).toArray(); // Get Earth from Sun
        // Reverse cartesian vector for Sun from Earth
        tvec.x = -KA[0]; tvec.y = -KA[1]; tvec.z = -KA[2];

	// J2000 coordinates of Sun from Earth
        var right_ascension_sun = tvec.RightAscension;
        var declination_sun = tvec.Declination * RADIANS_MULTIPLE;

        var HA = (Math.sin(theta_S) - (Math.sin(latitude) * Math.sin(declination_sun))) /
	         (Math.cos(latitude) * Math.cos(declination_sun));

        if (HA >-1.0 && HA <1.0) { 
            HA = Math.acos(HA) / RADIANS_MULTIPLE; // HA in degrees
        } else { 
	            return "//N/A at this latitude//";        // a bust! no solution for this latitude for today or tomorrow.
        }

        if (dawn_or_dusk) HA = -HA;		  // if for dawn, invert the position of the sun.
	
	var LMST = Util.Julian_GMST(JD) + (Telescope.SiteLongitude / 15.0);

        JD = JD + (((HA / 15.0) - Util.HourAngle12(right_ascension_sun, LMST)) / 24.0);
	
	if (Math.abs(JD - JDPrev) < JULIAN_SECOND) {
	    break;                                // Convergence to ~1 second accuracy breaks out of while loop
	}

        JDPrev = JD;
    }  while(true);
    
    KT = null; tvec = null ;		          // Explicitly release these

	return new Date(Util.Julian_Date(JD));
}

if (Util.Julian_Date(Util.SysJulianDate) > this_twilight(SUNSET, DAWN, TODAY)) {
    Response.write("It's After Dawn!\n");
    Response.write("Here are the //coming// day's sunset and sunrise times for //" +  Prefs.SiteName + "//\n" );
    Response.write("|>|!Dusk|" + ">|!Dawn|");
    Response.write("\n| Sunset is| " + this_twilight(SUNSET, DUSK, TOMORROW) + " | " + this_twilight(SUNSET, DAWN, TOMORROW) + " |is Sunrise |");
    Response.write("\n| Civil twilight ends| " + this_twilight(CIVIL, DUSK, TOMORROW) + " | " + this_twilight(CIVIL, DAWN, TOMORROW) + " |begins Civil twilight |");
    Response.write("\n| Nautical twilight ends| " + this_twilight(NAUTICAL, DUSK, TOMORROW) + " | " + this_twilight(NAUTICAL, DAWN, TOMORROW) + " |begins Nautical twilight |");
    Response.write("\n| Amateur twilight ends| " + this_twilight(AMATEUR, DUSK, TOMORROW) + " | " + this_twilight(AMATEUR, DAWN, TOMORROW) + " |begins Amateur twilight |");
    Response.write("\n| Astronomical twilight ends| " + this_twilight(ASTRONOMICAL, DUSK, TOMORROW) + " | " + this_twilight(ASTRONOMICAL, DAWN, TOMORROW) + " |begins Astronomical twilight |");
} else {
    Response.write("Here are //today's//  sunset and //tomorrow's// sunrise times for //" +  Prefs.SiteName + "//\n" );
    Response.write("|>|!Dusk|" + ">|!Dawn|");
    Response.write("\n| Sunset is| " + this_twilight(SUNSET, DUSK, TODAY) + " | " + this_twilight(SUNSET, DAWN, TODAY) + " |is Sunrise |");
    Response.write("\n| Civil twilight ends| " + this_twilight(CIVIL, DUSK, TODAY) + " | " + this_twilight(CIVIL, DAWN, TODAY) + " |begins Civil twilight |");
    Response.write("\n| Nautical twilight ends| " + this_twilight(NAUTICAL, DUSK, TODAY) + " | " + this_twilight(NAUTICAL, DAWN, TODAY) + " |begins Nautical twilight |");
    Response.write("\n| Amateur twilight ends| " + this_twilight(AMATEUR, DUSK, TODAY) + " | " + this_twilight(AMATEUR, DAWN, TODAY) + " |begins Amateur twilight |");
    Response.write("\n| Astronomical twilight ends| " + this_twilight(ASTRONOMICAL, DUSK, TODAY) + " | " + this_twilight(ASTRONOMICAL, DAWN, TODAY) + " |begins Astronomical twilight |");
 }

Response.write("\n----\n");	
Response.Write("//~~Contributed by Dick Berg, author of the [[ACP Reference Guide|http://solo.dc3.com/ar/ACPRefGuide.html]]~~//\n");
Response.Write("//^^Computational improvements by Robert Wahlstrom and Erik Young^^//");
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