Added the getTimesAtAltitude method

Author: MatthiasKunnen

README.md

@@ -79,6 +79,17 @@ Adds a custom time when the sun reaches the given angle to results returned by `
 
 `SunCalc.times` property contains all currently defined times.
 
+```javascript
+SunCalc.getTimesAtAltitude(/*Date*/ timeAndDate, /*Number*/ angleInDegrees, /*Number*/ latitude, /*Number*/ longitude)
+```
+
+Returns an object containing the times at which the sun will reach the given altitude at the given
+location. The object contains the following properties (each is a `Date` object):
+
+| Property        | Description                                                              |
+| --------------- | ------------------------------------------------------------------------ |
+| `rise`          | The time at which the sun will reach the altitude when it is rising      |
+| `set`           | The time at which the sun will reach the altitude when it is setting     |
 
 ### Sun position
 

suncalc.js

@@ -173,6 +173,30 @@ SunCalc.getTimes = function (date, lat, lng) {
     return result;
 };
 
+SunCalc.getTimesAtAltitude = function (date, altitude, lat, lng) {
+
+    var lw = rad * -lng,
+        phi = rad * lat,
+
+        d = toDays(date),
+        n = julianCycle(d, lw),
+        ds = approxTransit(0, lw, n),
+
+        M = solarMeanAnomaly(ds),
+        L = eclipticLongitude(M),
+        dec = declination(L, 0),
+
+        Jnoon = solarTransitJ(ds, M, L),
+
+        Jset = getSetJ(altitude * rad, lw, phi, dec, n, M, L),
+        Jrise = Jnoon - (Jset - Jnoon);
+
+    return {
+        rise: fromJulian(Jrise),
+        set: fromJulian(Jset),
+    };
+};
+
 
 // moon calculations, based on http://aa.quae.nl/en/reken/hemelpositie.html formulas
 

test.js

@@ -44,6 +44,13 @@ t.test('getTimes returns sun phases for the given date and location', function (
     t.end();
 });
 
+t.test('getTimesAtAltitude returns the correct time for the given date and location', function (t) {
+    var times = SunCalc.getTimesAtAltitude(date, -6, lat, lng);
+
+    t.equal(new Date(testTimes.dawn).toUTCString(), times.rise.toUTCString());
+    t.end();
+});
+
 t.test('getMoonPosition returns moon position data given time and location', function (t) {
     var moonPos = SunCalc.getMoonPosition(date, lat, lng);