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jonyrock/terminator.js

Created May 9, 2017 16:18
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terminator.js
/*
Terminator.js -- Overlay day/night region on a Leaflet map
based on https://github.com/joergdietrich/Leaflet.Terminator
*/
/*
Calculate the present UTC Julian Date. Function is valid after
the beginning of the UNIX epoch 1970-01-01 and ignores leap
seconds.
*/
Date.prototype.getJulian = function() {
return (this / 86400000) + 2440587.5;
}
/*
Calculate Greenwich Mean Sidereal Time according to
http://aa.usno.navy.mil/faq/docs/GAST.php
*/
Date.prototype.getGMST = function() {
var julianDay = this.getJulian();
var d = julianDay - 2451545.0;
// Low precision equation is good enough for our purposes.
return (18.697374558 + 24.06570982441908 * d) % 24;
}
export const Terminator = L.Polygon.extend({
options: {
color: '#00',
opacity: 0.5,
fillColor: '#00',
fillOpacity: 0.5,
resolution: 2
},
initialize: function(options) {
this.version = '0.1.0';
this._R2D = 180 / Math.PI;
this._D2R = Math.PI / 180;
L.Util.setOptions(this, options);
var latLng = this._compute(this.options.time || null)
this.setLatLngs(latLng);
},
setTime: function(date) {
console.log('set time');
this.options.time = date;
var latLng = this._compute(date || null)
this.setLatLngs(latLng);
},
/*
Compute the position of the Sun in ecliptic coordinates at julianDay.
Following http://en.wikipedia.org/wiki/Position_of_the_Sun
*/
_sunEclipticPosition: function(julianDay) {
// Days since start of J2000.0
var n = julianDay - 2451545.0;
// mean longitude of the Sun
var L = 280.460 + 0.9856474 * n;
L %= 360;
// mean anomaly of the Sun
var g = 357.528 + 0.9856003 * n;
g %= 360;
// ecliptic longitude of Sun
var lambda = L + 1.915 * Math.sin(g * this._D2R) +
0.02 * Math.sin(2 * g * this._D2R);
// distance from Sun in AU
var R = 1.00014 - 0.01671 * Math.cos(g * this._D2R) -
0.0014 * Math.cos(2 * g * this._D2R);
return {
"lambda": lambda,
"R": R
};
},
/*
Following the short term expression in
http://en.wikipedia.org/wiki/Axial_tilt#Obliquity_of_the_ecliptic_.28Earth.27s_axial_tilt.29
*/
_eclipticObliquity: function(julianDay) {
var n = julianDay - 2451545.0;
// Julian centuries since J2000.0
var T = n / 36525;
var epsilon = 23.43929111 -
T * (46.836769 / 3600 -
T * (0.0001831 / 3600 +
T * (0.00200340 / 3600 -
T * (0.576e-6 / 3600 -
T * 4.34e-8 / 3600))));
return epsilon;
},
/*
Compute the Sun's equatorial position from its ecliptic position.
Inputs are expected in degrees. Outputs are in degrees as well.
*/
_sunEquatorialPosition: function(sunEclLng, eclObliq) {
var alpha = Math.atan(Math.cos(eclObliq * this._D2R) *
Math.tan(sunEclLng * this._D2R)) * this._R2D;
var delta = Math.asin(Math.sin(eclObliq * this._D2R) *
Math.sin(sunEclLng * this._D2R)) * this._R2D;
var lQuadrant = Math.floor(sunEclLng / 90) * 90;
var raQuadrant = Math.floor(alpha / 90) * 90;
alpha = alpha + (lQuadrant - raQuadrant);
return {
"alpha": alpha,
"delta": delta
};
},
/*
Compute the hour angle of the sun for a longitude on Earth.
Return the hour angle in degrees.
*/
_hourAngle: function(lng, sunPos, gst) {
var lst = gst + lng / 15;
return lst * 15 - sunPos.alpha;
},
/*
For a given hour angle and sun position, compute the
latitude of the terminator in degrees.
*/
_latitude: function(ha, sunPos) {
var lat = Math.atan(-Math.cos(ha * this._D2R) /
Math.tan(sunPos.delta * this._D2R)) * this._R2D;
return lat;
},
_compute: function(time) {
if (time == null) {
var today = new Date();
} else {
var today = new Date(time);
}
var julianDay = today.getJulian();
var gst = today.getGMST();
var latLng = [];
var ha, lat;
var sunEclPos = this._sunEclipticPosition(julianDay);
var eclObliq = this._eclipticObliquity(julianDay);
var sunEqPos = this._sunEquatorialPosition(sunEclPos.lambda, eclObliq);
for (var i = 0; i <= 720 * this.options.resolution; i++) {
var lng = -360 + i / this.options.resolution;
ha = this._hourAngle(lng, sunEqPos, gst);
var lat = this._latitude(ha, sunEqPos);
latLng[i + 1] = [lat, lng];
}
if (sunEqPos.delta < 0) {
latLng[0] = [90, -360];
latLng[latLng.length] = [90, 360];
} else {
latLng[0] = [-90, -360];
latLng[latLng.length] = [-90, 360];
}
return latLng;
}
});
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