suncalc.js

/*
 (c) 2011-2015, Vladimir Agafonkin
 SunCalc is a JavaScript library for calculating sun position and light phases.
 https://github.com/mourner/suncalc
*/

var suncalc = (function () {
  'use strict';

// shortcuts for easier to read formulas

  var PI = Math.PI,
    sin = Math.sin,
    cos = Math.cos,
    tan = Math.tan,
    asin = Math.asin,
    atan = Math.atan2,
    rad = PI/180;

// sun calculations are based on http://aa.quae.nl/en/reken/zonpositie.html formulas


// date/time constants and conversions

  var dayMs = 1000*60*60*24,
    J1970 = 2440588,
    J2000 = 2451545;

  function toJulian(date) {
    return date.valueOf()/dayMs - 0.5 + J1970;
  }

  function toDays(date) {
    return toJulian(date) - J2000;
  }


// general calculations for position

  var e = rad*23.4397; // obliquity of the Earth

  function rightAscension(l, b) {
    return atan(sin(l)*cos(e) - tan(b)*sin(e), cos(l));
  }

  function declination(l, b) {
    return asin(sin(b)*cos(e) + cos(b)*sin(e)*sin(l));
  }

  function azimuth(H, phi, dec) {
    return atan(sin(H), cos(H)*sin(phi) - tan(dec)*cos(phi));
  }

  function altitude(H, phi, dec) {
    return asin(sin(phi)*sin(dec) + cos(phi)*cos(dec)*cos(H));
  }

  function siderealTime(d, lw) {
    return rad*(280.16 + 360.9856235*d) - lw;
  }


// general sun calculations

  function solarMeanAnomaly(d) {
    return rad*(357.5291 + 0.98560028*d);
  }

  function eclipticLongitude(M) {

    var C = rad*(1.9148*sin(M) + 0.02*sin(2*M) + 0.0003*sin(3*M)), // equation of center
      P = rad*102.9372; // perihelion of the Earth

    return M + C + P + PI;
  }

  function sunCoords(d) {

    var M = solarMeanAnomaly(d),
      L = eclipticLongitude(M);

    return {
      dec: declination(L, 0),
      ra: rightAscension(L, 0)
    };
  }

// calculates sun position for a given date and latitude/longitude

  return function(date, lat, lng) {

    var lw = rad* -lng,
      phi = rad*lat,
      d = toDays(date),

      c = sunCoords(d),
      H = siderealTime(d, lw) - c.ra;

    return {
      azimuth: azimuth(H, phi, c.dec),
      altitude: altitude(H, phi, c.dec)
    };
  };

}());