add solar.sin_elevation, fix solar.declination

lib/solar.js

@@ -67,6 +67,15 @@ function _distance(t) {
   return D0 + D1 * cos_g + D2 * cos_2g;
 }
 
+function _sin_declination(t) {
+  return Math.sin(_longitude(t)) * Math.sin(_obliquity(t));
+}
+
+function _right_ascension(t) {
+  const l = _longitude(t);
+  return Math.atan2(Math.sin(l) * Math.cos(_obliquity(t)), Math.cos(l));
+}
+
 // solar transit time
 const J0 = 0.0009; // Leap second correction
 function _julian_cycle(t, lon) {
@@ -87,7 +96,7 @@ function _transit(t, lon) {
 }
 
 function _altitude(sin_h0, rise, t, lat, lon) {
-  const sin_dec = Math.sin(_longitude(t)) * Math.sin(_obliquity(t));
+  const sin_dec = _sin_declination(t);
   const cos_dec = Math.sqrt(1 - sin_dec * sin_dec);
 
   return _transit(t, lon) + rise * Math.acos(
@@ -121,14 +130,21 @@ function distance(ms) {
 }
 
 function declination(ms) {
-  const t = julian.to(ms);
-  return Math.sin(_longitude(t)) * Math.sin(_obliquity(t));
+  return Math.asin(_sin_declination(julian.to(ms)));
 }
 
 function right_ascension(ms) {
+  return _right_ascension(julian.to(ms));
+}
+
+function sin_elevation(ms, lat, lon) {
   const t = julian.to(ms);
-  const l = _longitude(t);
-  return Math.atan2(Math.sin(l) * Math.cos(_obliquity(t)), Math.cos(l));
+  const sin_lat = Math.sin(lat * RAD);
+  const cos_lat = Math.cos(lat * RAD);
+  const sin_dec = _sin_declination(t);
+  const cos_dec = Math.sqrt(1 - sin_dec * sin_dec);
+  const hour_angle = sidereal.local(t, lon * RAD) - _right_ascension(t);
+  return sin_lat * sin_dec + cos_lat * cos_dec * Math.cos(hour_angle);
 }
 
 // Solar position classes. Helpful if you need to cache results!
@@ -230,6 +246,7 @@ exports.obliquity = obliquity;
 exports.distance = distance;
 exports.declination = declination;
 exports.right_ascension = right_ascension;
+exports.sin_elevation = sin_elevation;
 exports.position = position;
 exports.transit = transit;
 exports.rise = rise;

package.json

@@ -1,6 +1,6 @@
 {
   "name": "astro",
-  "version": "2.0.3",
+  "version": "2.0.4",
   "description": "Solar and lunar position calculations",
   "main": "index.js",
   "devDependencies": {