pyephem:无法计算极地地区的日出/日落

Question

我正在尝试使用 pyephem 计算日出和日落，但该算法似乎从未在极地地区收敛？

观察下面的示例代码。它以 10 分钟为增量迭代一整年，询问下一次日出和日落。 pyephem 总是返回一个 AlwaysUpError 或 NeverUpError，但一年中太阳必须至少升起和落下一次吗？

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

要么是我错误地使用了 api，pyephem 中存在错误，要么是我误解了一些基本的东西。有什么帮助吗？

Answer 1

我怀疑存在某种不正确的缓存。考虑:

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()) 

产生:

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 

与 USNO 结果匹配的分钟:

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

另请参阅链接问题中我的相关提示。