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Pyephem: cannot calculate sunrise / set for polar regions

I am trying to calculate sunrises and sunsets using pyephem, but the algorithm never converges for polar regions?

follow the code example below. it repeats itself after a whole year with a 10-minute step, asking for the next dawn and sunset. pyephem always returns with AlwaysUpError or NeverUpError, but surely the sun should rise and set at least once during the year?

import ephem
from datetime import datetime, timedelta

obs = ephem.Observer()
obs.lat = '89:30'
obs.long = '0'

start = datetime(2011, 1, 1)
end = datetime(2012, 1, 1)
step = timedelta(minutes=10)

sun = ephem.Sun()

timestamp = start
while timestamp < end:
    obs.date = timestamp

    try:
        print obs.next_rising(sun)
    except (ephem.AlwaysUpError, ephem.NeverUpError):
        pass

    try:
        print obs.next_setting(sun)
    except (ephem.AlwaysUpError, ephem.NeverUpError):
        pass

    try:
        print obs.previous_rising(sun)
    except (ephem.AlwaysUpError, ephem.NeverUpError):
        pass

    try:
        print obs.previous_setting(sun)
    except (ephem.AlwaysUpError, ephem.NeverUpError):
        pass

    timestamp += step

either I use the api incorrectly, there is an error in pyephem, or I misunderstand something fundamental. any help?

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3 answers

I suspect some kind of improper caching. Consider:

import ephem 
atlanta = ephem.Observer() 
atlanta.pressure = 0 
atlanta.horizon = '-0:34' 
atlanta.lat, atlanta.lon = '89:30', '0' 
atlanta.date = '2011/03/18 12:00' 
print atlanta.previous_rising(ephem.Sun()) 
print atlanta.next_setting(ephem.Sun()) 
atlanta.date = '2011/03/19 12:00' 
print atlanta.previous_rising(ephem.Sun()) 
print atlanta.next_setting(ephem.Sun()) 
atlanta.date = '2011/03/20 12:00' 
print atlanta.previous_rising(ephem.Sun()) 
# print atlanta.next_setting(ephem.Sun()) 
atlanta.date = '2011/09/24 12:00' 
# print atlanta.previous_rising(ephem.Sun()) 
print atlanta.next_setting(ephem.Sun()) 
atlanta.date = '2011/09/25 12:00' 
print atlanta.previous_rising(ephem.Sun()) 
print atlanta.next_setting(ephem.Sun()) 
atlanta.date = '2011/09/26 12:00' 
print atlanta.previous_rising(ephem.Sun()) 
print atlanta.next_setting(ephem.Sun())

which gives:

2011/3/18 07:49:34 
2011/3/18 17:44:50 
2011/3/19 05:04:49 
2011/3/19 21:49:23 
2011/3/20 01:26:02 
2011/9/24 19:59:09 
2011/9/25 04:57:21 
2011/9/25 17:14:10 
2011/9/26 08:37:25 
2011/9/26 14:03:20

which corresponds to a minute with USNO results:

https://raw.github.com/barrycarter/bcapps/master/db/srss-895.txt

. .

+1

( "sort | uniq -c" ):

260 2011/3/17 11:32:31
469 2011/3/17 13:42:56
184 2011/3/18 07:25:56
350 2011/3/18 18:13:15
191 2011/3/19 04:41:42
346 2011/9/24 20:25:13
337 2011/9/25 04:27:45
214 2011/9/25 17:36:10
166 2011/9/26 08:00:59
254 2011/9/26 14:37:06

, ? :

https://raw.github.com/barrycarter/bcapps/master/playground4.py

( , (-34 -50 ).

+1

I found, using the option startfor obs.next_rising(), etc., gives better results. however, as before, apparently, he sometimes misses certain transitions; the enhancements he found do not always connect to the corresponding set.

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