corrected get_refraction_correction

Corrected approximation according to NREL's C code (http://www.nrel.gov/midc/spa/) Now the function is only applied if the sun is not close to horizon
tuxiano · Jan 9, 2015 · cb89528 · cb89528
1 parent 61476a2
commit cb89528
Showing 1 changed file with 17 additions and 3 deletions.
diff --git a/pysolar/solar.py b/pysolar/solar.py
@@ -286,10 +286,24 @@ def get_sun_earth_distance(jme):
     return get_coeff(jme, constants.sun_earth_distance_coeffs) / 1e8
 
 def get_refraction_correction(pressure, temperature, topocentric_elevation_angle):
+
+    #function and default values according to original NREL SPA C code
+    #http://www.nrel.gov/midc/spa/ 
+
+    sun_radius = 0.26667  
+    atmos_refract = 0.5667
+
+    del_e = 0.0   
     tea = topocentric_elevation_angle
-    a = pressure * 2.830 * 1.02
-    b = 1010.0 * temperature * 60.0 * math.tan(math.radians(tea + (10.3/(tea + 5.11))))
-    return a / b
+
+    #approximation only valid if sun is not close to horizon
+    if (tea >= -1.0*(sun_radius + atmos_refract))   
+
+        a = pressure * 2.830 * 1.02
+        b = 1010.0 * temperature * 60.0 * math.tan(math.radians(tea + (10.3/(tea + 5.11))))
+        del_e = a / b
+
+    return del_e
 
 def get_solar_time(longitude_deg, when):
     "returns solar time in hours for the specified longitude and time," \