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-rw-r--r--firmament/sun.py32
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diff --git a/firmament/sun.py b/firmament/sun.py
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+++ b/firmament/sun.py
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+import numpy as np
+from datetime import datetime
+
+## Sun direction depending on time and place
+# As described in Appendix D of "ME 4131 Thermal Environmental Engineering Laboratory Manual"
+
+def sun_declination(time):
+ day_of_year = time.timetuple().tm_yday
+ return 23.45 * np.sin(np.radians((360/365)*(284+day_of_year)))
+
+def equation_of_time(time):
+ day_of_year = time.timetuple().tm_yday
+ b = np.radians(360*(day_of_year-81)/364)
+ return 0.165*np.sin(2*b) - 0.126*np.cos(b) - 0.025*np.sin(b)
+
+def sun_direction(lat, lon, time, time_diff, summertime_shift = 0):
+ lon_std = time_diff * 15
+ clock_time = time.hour + time.minute/60
+ local_solar_time = clock_time + (1/15)*(lon - lon_std) + equation_of_time(time) - summertime_shift
+ hour_angle = 15*(local_solar_time - 12)
+
+ l = np.radians(lat)
+ h = np.radians(hour_angle)
+ d = np.radians(sun_declination(time))
+
+ altitude = np.arcsin(np.cos(l) * np.cos(h) * np.cos(d) + np.sin(l) * np.sin(d))
+ azimuth = np.arccos((np.cos(d) * np.sin(l) * np.cos(h) - np.sin(d) * np.cos(l)) / np.cos(altitude))
+
+ if h < 0:
+ return (altitude, azimuth)
+ else:
+ return (altitude, -azimuth)