144 lines
4.3 KiB
JavaScript
144 lines
4.3 KiB
JavaScript
/* Terminator.js -- Overlay day/night region on a Leaflet map */
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Date.prototype.getJulian = function() {
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/* Calculate the present UTC Julian Date. Function is valid after
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* the beginning of the UNIX epoch 1970-01-01 and ignores leap
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* seconds. */
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return (this / 86400000) + 2440587.5;
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}
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Date.prototype.getGMST = function() {
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/* Calculate Greenwich Mean Sidereal Time according to
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http://aa.usno.navy.mil/faq/docs/GAST.php */
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var julianDay = this.getJulian();
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var d = julianDay - 2451545.0;
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// Low precision equation is good enough for our purposes.
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return (18.697374558 + 24.06570982441908 * d) % 24;
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}
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L.Terminator = L.Polygon.extend({
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options: {
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color: '#00',
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opacity: 0.5,
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fillColor: '#00',
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fillOpacity: 0.5,
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resolution: 2
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},
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initialize: function(options) {
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this.version = '0.1.0';
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this._R2D = 180 / Math.PI;
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this._D2R = Math.PI / 180;
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L.Util.setOptions(this, options);
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var latLng = this._compute(this.options.time || null)
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this.setLatLngs(latLng);
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},
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setTime: function(date) {
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this.options.time = date;
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var latLng = this._compute(date || null)
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this.setLatLngs(latLng);
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},
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_sunEclipticPosition: function(julianDay) {
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/* Compute the position of the Sun in ecliptic coordinates at
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julianDay. Following
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http://en.wikipedia.org/wiki/Position_of_the_Sun */
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// Days since start of J2000.0
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var n = julianDay - 2451545.0;
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// mean longitude of the Sun
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var L = 280.460 + 0.9856474 * n;
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L %= 360;
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// mean anomaly of the Sun
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var g = 357.528 + 0.9856003 * n;
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g %= 360;
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// ecliptic longitude of Sun
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var lambda = L + 1.915 * Math.sin(g * this._D2R) +
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0.02 * Math.sin(2 * g * this._D2R);
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// distance from Sun in AU
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var R = 1.00014 - 0.01671 * Math.cos(g * this._D2R) -
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0.0014 * Math.cos(2 * g * this._D2R);
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return {"lambda": lambda, "R": R};
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},
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_eclipticObliquity: function(julianDay) {
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// Following the short term expression in
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// http://en.wikipedia.org/wiki/Axial_tilt#Obliquity_of_the_ecliptic_.28Earth.27s_axial_tilt.29
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var n = julianDay - 2451545.0;
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// Julian centuries since J2000.0
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var T = n / 36525;
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var epsilon = 23.43929111 -
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T * (46.836769 / 3600
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- T * (0.0001831 / 3600
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+ T * (0.00200340 / 3600
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- T * (0.576e-6 / 3600
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- T * 4.34e-8 / 3600))));
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return epsilon;
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},
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_sunEquatorialPosition: function(sunEclLng, eclObliq) {
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/* Compute the Sun's equatorial position from its ecliptic
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* position. Inputs are expected in degrees. Outputs are in
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* degrees as well. */
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var alpha = Math.atan(Math.cos(eclObliq * this._D2R)
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* Math.tan(sunEclLng * this._D2R)) * this._R2D;
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var delta = Math.asin(Math.sin(eclObliq * this._D2R)
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* Math.sin(sunEclLng * this._D2R)) * this._R2D;
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var lQuadrant = Math.floor(sunEclLng / 90) * 90;
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var raQuadrant = Math.floor(alpha / 90) * 90;
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alpha = alpha + (lQuadrant - raQuadrant);
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return {"alpha": alpha, "delta": delta};
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},
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_hourAngle: function(lng, sunPos, gst) {
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/* Compute the hour angle of the sun for a longitude on
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* Earth. Return the hour angle in degrees. */
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var lst = gst + lng / 15;
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return lst * 15 - sunPos.alpha;
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},
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_latitude: function(ha, sunPos) {
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/* For a given hour angle and sun position, compute the
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* latitude of the terminator in degrees. */
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var lat = Math.atan(-Math.cos(ha * this._D2R) /
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Math.tan(sunPos.delta * this._D2R)) * this._R2D;
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return lat;
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},
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_compute: function(time) {
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if (time == null)
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var today = new Date();
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else
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var today = new Date(time);
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var julianDay = today.getJulian();
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var gst = today.getGMST();
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var latLng = [];
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var ha, lat;
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var sunEclPos = this._sunEclipticPosition(julianDay);
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var eclObliq = this._eclipticObliquity(julianDay);
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var sunEqPos = this._sunEquatorialPosition(sunEclPos.lambda, eclObliq);
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for (var i = 0; i <= 720 * this.options.resolution; i++) {
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lng = -360 + i / this.options.resolution;
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ha = this._hourAngle(lng, sunEqPos, gst);
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lat = this._latitude(ha, sunEqPos);
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latLng[i+1] = [lat, lng];
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}
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if (sunEqPos.delta < 0) {
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latLng[0] = [90, -360];
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latLng[latLng.length] = [90, 360];
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} else {
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latLng[0] = [-90, -360];
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latLng[latLng.length] = [-90, 360];
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}
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return latLng;
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}
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});
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L.terminator = function(options) {
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return new L.Terminator(options);
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};
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