fixed bug on calculation of astronomical refraction

the formula 16.4 of "Astronomical Algorithms" 2nd edition is now
translated correctly to radians. additionally the calculation works
also for negative altitudes now.

suncalc.js

@@ -43,6 +43,14 @@ function altitude(H, phi, dec) { return asin(sin(phi) * sin(dec) + cos(phi) * co
 
 function siderealTime(d, lw) { return rad * (280.16 + 360.9856235 * d) - lw; }
 
+function astroRefraction(h) {
+    if (h < 0) // the following formula works for positive altitudes only.
+        h = 0; // if h = -0.08901179 a div/0 would occur.
+
+    // formula 16.4 of "Astronomical Algorithms" 2nd edition by Jean Meeus (Willmann-Bell, Richmond) 1998.
+    // 1.02 / tan(h + 10.26 / (h + 5.10)) h in degrees, result in arc minutes -> converted to rad:
+    return 0.0002967 / Math.tan(h + 0.00312536 / (h + 0.08901179));
+}
 
 // general sun calculations
 
@@ -197,8 +205,7 @@ SunCalc.getMoonPosition = function (date, lat, lng) {
         // formula 14.1 of "Astronomical Algorithms" 2nd edition by Jean Meeus (Willmann-Bell, Richmond) 1998.
         pa = atan(sin(H), tan(phi) * cos(c.dec) - sin(c.dec) * cos(H));
 
-    // altitude correction for refraction
-    h = h + rad * 0.017 / tan(h + rad * 10.26 / (h + rad * 5.10));
+    h = h + astroRefraction(h); // altitude correction for refraction
 
     return {
         azimuth: azimuth(H, phi, c.dec),

test.js

@@ -48,7 +48,7 @@ t.test('getMoonPosition returns moon position data given time and location', fun
     var moonPos = SunCalc.getMoonPosition(date, lat, lng);
 
     t.ok(near(moonPos.azimuth, -0.9783999522438226), 'azimuth');
-    t.ok(near(moonPos.altitude, 0.006969727754891917), 'altitude');
+    t.ok(near(moonPos.altitude, 0.014551482243892251), 'altitude');
     t.ok(near(moonPos.distance, 364121.37256256194), 'distance');
     t.end();
 });
@@ -65,8 +65,8 @@ t.test('getMoonIllumination returns fraction and angle of moon\'s illuminated li
 t.test('getMoonTimes returns moon rise and set times', function (t) {
     var moonTimes = SunCalc.getMoonTimes(new Date('2013-03-04UTC'), lat, lng, true);
 
-    t.equal(moonTimes.rise.toUTCString(), 'Mon, 04 Mar 2013 23:57:55 GMT');
-    t.equal(moonTimes.set.toUTCString(), 'Mon, 04 Mar 2013 07:28:41 GMT');
+    t.equal(moonTimes.rise.toUTCString(), 'Mon, 04 Mar 2013 23:54:29 GMT');
+    t.equal(moonTimes.set.toUTCString(), 'Mon, 04 Mar 2013 07:47:58 GMT');
 
     t.end();
 });