Check if night hours (TimeWithinDay; AreaSolarRadiation)

Question

I am creating a script that uses AreaSolarRadiation. Currently arcpy gives a cryptic error during the night hours.

arcgisscripting.ExecuteError: ERROR 999999: Error executing function.
Failed to open raster dataset
Failed to execute (AreaSolarRadiation)

Is there a way to simply check to know when it is night hours so I can skip running the tool? Currently I just catch the cryptic error and skip it. The script calculates Solar Radiation every hour over several days. It would be nice to have a quick check to save computation time during night hours.

import arcpy
from arcpy.sa import *
import datetime

arcpy.env.workspace = r'C:\Workspace'
arcpy.CheckOutExtension('Spatial')

print('Begin Script')

date_time = datetime.datetime.strptime('2014-01-01', '%Y-%m-%d')
day_of_year = date_time.timetuple().tm_yday #(Jan 1 = 1)
start = 12 # Hour to start calculation
end = 13 # Hour to end
twd = TimeWithinDay(day_of_year, start, end) 
sky_size = 200 
dem = r'C:\Workspace\dem.tif' # Any DEM file (use a small one for testing)

# Works during the day
out_solar_radiation = AreaSolarRadiation(dem, '', sky_size, twd)
print(out_solar_radiation)

start = 2
end = 3
twd = TimeWithinDay(day_of_year, start, end) 

# Does not work at night
out_solar_radiation = AreaSolarRadiation(dem, '', sky_size, twd)
print out_solar_radiation
print('End Script')

Using ArcGIS 10.3

When the workspace is a place arcpy can write to I get a different error

arcgisscripting.ExecuteError: ERROR 010036: Error in creating the sun map.
ERROR 010003: The time range is at night when sun light is not available
Failed to execute (AreaSolarRadiation)

Esri help pages don't have very helpful information though.

http://resources.arcgis.com/en/help/main/10.1/index.html#//00vq0000000n010036 http://resources.arcgis.com/en/help/main/10.1/index.html#//00vq00000002010003

Tool dialogue and error screenshots

Answer 1

What about using datetime? Like:

from datetime import datetime as dt
now_in_hours = dt.now().time().hour
if now_in_hours in range(6, 18):
    do process...

Update: You can retrieve the current sunrise and sunset times based on location from a web service like: http://sunrise-sunset.org/ and then use the result in the above if statement:

import urllib
import json

def sun (lat, lon):
    url = 'http://api.sunrise-sunset.org/json?lat=%f&lng=%f' % (lat, lon)
    result= json.loads(urllib.urlopen(url).read())

    sunrise = result['results']['sunrise']
    sunset = result['results']['sunset']
    return (sunrise, sunset)

lat, lon =  46.05145, -14.506053
print sun(lat,lon)   ## (u'7:22:07 AM', u'6:03:26 PM')