sun-position

Python code for computing sun position based on the algorithms from "Solar position algorithm for solar radiation applications" by Ibrahim Reda and Afshin Anreas, Solar Energy (2004). The algorithm calculates "the solar zenith and azimuth angles in the period from the year −2000 to 6000, with uncertainties of ±0.0003°". See http://dx.doi.org/10.1016/j.solener.2003.12.003 for more information.

In this code, the latitude and longitude are positive for North and East, respectively. The azimuth angle is 0 at North and positive towards the east. The zenith angle is 0 at vertical and positive towards the horizon.

Example usage on the command line:

$ sunposition.py --help
usage: sunposition.py [-h] [-t,--time T] [-lat,--latitude LAT]
                      [-lon,--longitude LON] [-e,--elevation ELEV]
                      [-T,--temperature TEMP] [-p,--pressure P] [-dt DT]

Compute sun position parameters given the time and location

optional arguments:
  -h, --help            show this help message and exit
  -t,--time T           "now" or date and time (UTC) in "YYYY-MM-DD
                        hh:mm:ss.ssssss" format or a (UTC) POSIX timestamp
  -lat,--latitude LAT   latitude, in decimal degrees, positive for north
  -lon,--longitude LON  longitude, in decimal degrees, positive for east
  -e,--elevation ELEV   elevation, in meters
  -T,--temperature TEMP
                        temperature, in degrees celcius
  -p,--pressure P       atmospheric pressure, in millibar
  -dt DT                difference between earth's rotation time (TT) and
                        universal time (UT1)
$ sunposition.py
Computing sun position at T = 2015-04-02 04:58:00.185177 + 0.0 s
Lat, Long, Elev = 51.48 deg, 0.0 deg, 0 m
T, P = 14.6 C, 1013.0 mbar
Results:
Azimuth, zenith = 74.0758398512 deg, 96.4040169751 deg
RA, dec, H = 11.2148787985 deg, 4.78553367225 deg, 253.554142824 deg

$ sunposition.py -t "1953-05-29 05:45:00" -lat 27.9881 -lon 86.9253 -e 8848
Computing sun position at T = 1953-05-29 05:45:00 + 0.0 s
Lat, Long, Elev = 27.9881 deg, 86.9253 deg, 8848.0 m
T, P = 14.6 C, 1013.0 mbar
Results:
Azimuth, zenith = 137.203450764 deg, 8.41097958239
RA, dec, H = 65.958212307 deg, 21.6806308847 deg, 353.859123093 deg

Example usage in code:

from pylab import *
from sunposition import sun_position
from datetime import datetime

#evaluate on a 2 degree grid
lon  = linspace(-180,180,181)
lat = linspace(-90,90,91)
LON, LAT = meshgrid(lon,lat)
#at the current time
now = datetime.utcnow()
az,zen = sun_position(now,LAT,LON,0)[:2] #discard RA, dec, H
#convert zenith to elevation
elev = 90 - zen
#convert azimuth to vectors
u, v = cos((90-az)*pi/180), sin((90-az)*pi/180)
#plot
figure()
imshow(elev,cmap=cm.CMRmap,origin='lower',vmin=-90,vmax=90,extent=(-180,180,-90,90))
s = slice(5,-1,5) # equivalent to 5:-1:5
quiver(lon[s],lat[s],u[s,s],v[s,s])
contour(lon,lat,elev,[0])
cb = colorbar()
cb.set_label('Elevation Angle (deg)')
gca().set_aspect('equal')
xticks(arange(-180,181,45)); yticks(arange(-90,91,45))

Citation: Ibrahim Reda, Afshin Andreas, Solar position algorithm for solar radiation applications, Solar Energy, Volume 76, Issue 5, 2004, Pages 577-589, ISSN 0038-092X, http://dx.doi.org/10.1016/j.solener.2003.12.003. Keywords: Global solar irradiance; Solar zenith angle; Solar azimuth angle; VSOP87 theory; Universal time; ΔUT1