astro.ephem.js

/*
	astro.ephem.js - get ephemerides

*/
(function () {

	// Create the main function
	function init(astrojs) {

		this.d2r = Math.PI/180;

		// Uses algorithm defined in Practical Astronomy (4th ed) by Peter Duffet-Smith and Jonathan Zwart
		this.sunPos = function(JD){
			var D,eg,wg,e,N,Mo,v,lon;
			D = (JD-2455196.5);	// Number of days since the epoch of 2010 January 0.0
			// Calculated for epoch 2010.0. If T is the number of Julian centuries since 1900 January 0.5 = (JD-2415020.0)/36525
			eg = 279.557208;	// mean ecliptic longitude in degrees = (279.6966778 + 36000.76892*T + 0.0003025*T*T)%360;
			wg = 283.112438;	// longitude of the Sun at perigee in degrees = 281.2208444 + 1.719175*T + 0.000452778*T*T;
			e = 0.016705;	// eccentricity of the Sun-Earth orbit in degrees = 0.01675104 - 0.0000418*T - 0.000000126*T*T;
			N = ((360/365.242191)*D)%360;
			if(N < 0) N += 360;
			Mo = (N + eg - wg)%360	// mean anomaly in degrees
			if(Mo < 0) Mo += 360;
			v = Mo + (360/Math.PI)*e*Math.sin(Mo*Math.PI/180);
			lon = v + wg;
			if(lon > 360) lon -= 360;
			return {lat:0, lon:lon, Mo:Mo, D:D, N:N}
		}


		// Uses algorithm defined in Practical Astronomy (4th ed) by Peter Duffet-Smith and Jonathan Zwart
		this.moonPos = function(JD,sun){
			var d2r,lo,Po,No,i,e,l,Mm,N,C,Ev,sinMo,Ae,A3,Mprimem,Ec,A4,lprime,V,lprimeprime,Nprime,lppNp,sinlppNp,y,x,lm,Bm;
			d2r = this.d2r;
			if(typeof JD=="undefined") return { status: -1 };
			if(typeof sun=="undefined") sun = this.sunPos(JD);
			lo = 91.929336;	// Moon's mean longitude at epoch 2010.0
			Po = 130.143076;	// mean longitude of the perigee at epoch
			No = 291.682547;	// mean longitude of the node at the epoch
			i = 5.145396;	// inclination of Moon's orbit
			e = 0.0549;	// eccentricity of the Moon's orbit
			l = (13.1763966*sun.D + lo)%360;
			if(l < 0) l += 360;
			Mm = (l - 0.1114041*sun.D - Po)%360;
			if(Mm < 0) Mm += 360;
			N = (No - 0.0529539*sun.D)%360;
			if(N < 0) N += 360;
			C = l-sun.lon;
			Ev = 1.2739*Math.sin((2*C-Mm)*d2r);
			sinMo = Math.sin(sun.Mo*d2r);
			Ae = 0.1858*sinMo;
			A3 = 0.37*sinMo;
			Mprimem = Mm + Ev -Ae - A3;
			Ec = 6.2886*Math.sin(Mprimem*d2r);
			A4 = 0.214*Math.sin(2*Mprimem*d2r);
			lprime = l + Ev + Ec -Ae + A4;
			V = 0.6583*Math.sin(2*(lprime-sun.lon)*d2r);
			lprimeprime = lprime + V;
			Nprime = N - 0.16*sinMo;
			lppNp = (lprimeprime-Nprime)*d2r;
			sinlppNp = Math.sin(lppNp);
			y = sinlppNp*Math.cos(i*d2r);
			x = Math.cos(lppNp);
			lm = Math.atan2(y,x)/d2r + Nprime;
			Bm = Math.asin(sinlppNp*Math.sin(i*d2r))/d2r;
			if(lm > 360) lm -= 360;
			return { lat:Bm, lon:lm, sun:sun };
		}
		
		return this;

	}

	astrojs.registerPackage({
		init: init,
		name: 'ephem',
		version: '0.1'
	});
	
})(astrojs);