add SunCalc.getMoonTimes, close #7

suncalc.js

@@ -231,6 +231,65 @@ SunCalc.getMoonIllumination = function (date) {
 };
 
 
+function hoursLater(date, h) {
+    return new Date(date.valueOf() + h * dayMs / 24);
+}
+
+// calculations for moon rise/set times are based on http://www.stargazing.net/kepler/moonrise.html article
+
+SunCalc.getMoonTimes = function (date, lat, lng) {
+    var t = new Date(date);
+    t.setHours(0);
+    t.setMinutes(0);
+    t.setSeconds(0);
+    t.setMilliseconds(0);
+
+    var hc = 0.133 * rad,
+        h0 = SunCalc.getMoonPosition(t, lat, lng).altitude - hc,
+        h1, h2, rise, set, a, b, xe, ye, d, roots, x1, x2, dx;
+
+    // go in 2-hour chunks, each time seeing if a 3-point quadratic curve crosses zero (which means rise or set)
+    for (var i = 1; i <= 24; i += 2) {
+        h1 = SunCalc.getMoonPosition(hoursLater(t, i), lat, lng).altitude - hc;
+        h2 = SunCalc.getMoonPosition(hoursLater(t, i + 1), lat, lng).altitude - hc;
+
+        a = (h0 + h2) / 2 - h1;
+        b = (h2 - h0) / 2;
+        xe = -b / (2 * a);
+        ye = (a * xe + b) * xe + h1;
+        d = b * b - 4 * a * h1;
+        roots = 0;
+
+        if (d >= 0) {
+            dx = Math.sqrt(d) / (Math.abs(a) * 2);
+            x1 = xe - dx;
+            x2 = xe + dx;
+            if (Math.abs(x1) <= 1) roots++;
+            if (Math.abs(x2) <= 1) roots++;
+            if (x1 < -1) x1 = x2;
+        }
+
+        if (roots === 1) {
+            if (h0 < 0) rise = i + x1;
+            else set = i + x1;
+
+        } else if (roots === 2) {
+            rise = i + (ye < 0 ? x2 : x1);
+            set = i + (ye < 0 ? x1 : x2);
+        }
+
+        if (rise && set) break;
+
+        h0 = h2;
+    }
+
+    return {
+        rise: hoursLater(t, rise),
+        set: hoursLater(t, set)
+    };
+};
+
+
 // export as AMD module / Node module / browser variable
 if (typeof define === 'function' && define.amd) define(SunCalc);
 else if (typeof module !== 'undefined') module.exports = SunCalc;