Add getRiseAndSetAtSolarAngle method

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.getRiseAndSetAtSolarAngle(/*Date*/ timeAndDate, /*Number*/ angle, /*Number*/ latitude, /*Number*/ longitude, /*Number (default=0)*/ elevation)
+```
+
+Returns an object containing the times at which the sun will reach the given angle at the given
+location and elevation. The object contains the following properties (each is a `Date` object):
+
+| Property        | Description                                                              |
+| --------------- | ------------------------------------------------------------------------ |
+| `rise`          | The time at which the sun will reach the given angle when it is rising   |
+| `set`           | The time at which the sun will reach the given angle when it is setting  |
 
 ### Sun position
 

suncalc.js

@@ -180,6 +180,46 @@ SunCalc.getTimes = function (date, lat, lng, height) {
     return result;
 };
 
+/**
+ * Get the time at which the sun will have a given apparent angle when rising and when setting.
+ *
+ * @param {Date} date The date to get the times for. Only the date part is important.
+ * @param {number} angle The angle of the sun relative to the Earth's horizon.
+ * @param {number} lat The latitude.
+ * @param {number} lng The longitude.
+ * @param {number} [elevation=0] The elevation of the observer in meters.
+ * @return {{set: Date, rise: Date}}
+ */
+SunCalc.getRiseAndSetAtSolarAngle = function (date, angle, lat, lng, elevation) {
+
+    elevation = elevation || 0;
+    angle = angle - astroRefraction(angle) * 100;
+
+    var lw = rad * -lng,
+        phi = rad * lat,
+
+        dh = observerAngle(elevation),
+
+        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),
+        h0 = (angle + dh) * rad,
+
+        Jset = getSetJ(h0, 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

@@ -61,6 +61,23 @@ t.test('getTimes adjusts sun phases when additionally given the observer height'
     t.end();
 });
 
+t.test('getRiseAndSetAtSolarAngle returns the correct time for the given date and location', function (t) {
+    var times = SunCalc.getRiseAndSetAtSolarAngle(date, 0, lat, lng);
+
+    t.equal(new Date(times.rise).toString(), new Date(testTimes.sunrise).toString());
+    t.equal(times.set.toString(), new Date(testTimes.sunset).toString());
+    t.end();
+});
+
+t.test('getRiseAndSetAtAngle adjusts sun phases when additionally given the observer\'s elevation', function (t) {
+    var times = SunCalc.getRiseAndSetAtSolarAngle(date, 0, lat, lng, height);
+
+    t.equal(new Date(heightTestTimes.sunrise).toUTCString(), times.rise.toUTCString());
+    t.equal(new Date(heightTestTimes.sunset).toUTCString(), times.set.toUTCString());
+
+    t.end();
+});
+
 t.test('getMoonPosition returns moon position data given time and location', function (t) {
     var moonPos = SunCalc.getMoonPosition(date, lat, lng);