我有一个函数可以返回给定位置的日出、日落、正午和黄昏时间。此函数使用 pyephem由于它已被弃用，我想我会重新编写函数以使用 skyfield。但是，skyfield 没有.previous_rising、.next_setting 或.next_transit 函数(至少我找不到)，这就是我与 pyephem 一起使用的。
skyfield 确实有函数 find_discrete ，它将在给定时间之间搜索以查找函数何时更改，因此我编写了以下内容来测试:
from datetime import datetime, timedelta
from skyfield import api, almanac
import pytz
ts = api.load.timescale()
planets = api.load('de421.bsp')
sun, earth = planets['sun'], planets['earth']
lat, lon = '44.59028 N', '104.71528 W' # UFO Mooring Site
tzn, elv = 'US/Mountain', 1559
tz = pytz.timezone(tzn)
loc = api.Topos(lat, lon, elevation_m=elv)
earth_loc = earth + loc
def civil_twil(time):
"returns true/false if sun is above -6 degrees"
alt, az, dis = earth_loc.at(time).observe(sun).apparent().altaz()
return alt > -6
t0 = ts.utc(datetime.now(tz))
t1 = ts.utc(tz.normalize(datetime.now(tz) + timedelta(1)))
civil_dawn = almanac.find_discrete(t0, t1, civil_twil)
但这只是给我一个错误，即该函数缺少“rough_period”属性，并且文档没有提及可能是什么。我猜它可能只对 almanac 类中定义的函数有效；但同样，它没有被提及。
那么，如何通过天空场找到/确定黄昏时间？
请您参考如下方法:
有一个example在文档中使用 find_discrete 函数查找日出和日落的值。请参阅 almanac.py 中的来源对于这个功能。这里唯一的问题是它仅在太阳顶部明显与地平线齐平时才计算。在下面的示例中，我将 sunrise_sunset 函数更改为 daylength 函数。有了这个，您可以为 daylength 函数提供所需的角度。
from skyfield import api, almanac
from datetime import datetime, timedelta
import pytz
from skyfield.nutationlib import iau2000b
DAYLENGTH_CENTER_HORIZON = 0.0
DAYLENGTH_TOP_HORIZON = 0.26667
DAYLENGTH_TOP_HORIZON_APPARENTLY = 0.8333
DAYLENGTH_CIVIL_TWILIGHT = 6.0
DAYLENGTH_NAUTICAL_TWILIGHT = 12.0
DAYLENGTH_ASTRONOMICAL_TWILIGHT = 18.0
def daylength(ephemeris, topos, degrees):
"""Build a function of time that returns the daylength.
The function that this returns will expect a single argument that is a
:class:`~skyfield.timelib.Time` and will return ``True`` if the sun is up
or twilight has started, else ``False``.
"""
sun = ephemeris['sun']
topos_at = (ephemeris['earth'] + topos).at
def is_sun_up_at(t):
"""Return `True` if the sun has risen by time `t`."""
t._nutation_angles = iau2000b(t.tt)
return topos_at(t).observe(sun).apparent().altaz()[0].degrees > -degrees
is_sun_up_at.rough_period = 0.5 # twice a day
return is_sun_up_at
ts = api.load.timescale()
planets = api.load('de421.bsp')
sun = planets['sun']
earth = planets['earth']
lat, lon = '44.59028 N', '104.71528 W' # UFO Mooring Site
tzn, elv = 'US/Mountain', 1559
tz = pytz.timezone(tzn)
loc = api.Topos(lat, lon, elevation_m=elv)
t0 = ts.utc(datetime.now(tz))
t1 = ts.utc(tz.normalize(datetime.now(tz) + timedelta(1)))
center_time, center_up = almanac.find_discrete(t0, t1, daylength(planets, loc,
DAYLENGTH_CENTER_HORIZON))
print('Sunrise Sunset center of sun is even with horizon:')
print(center_time.utc_iso(), center_up)
apparent_top_time, apparent_top_up = almanac.find_discrete(t0, t1,
daylength(planets, loc, DAYLENGTH_TOP_HORIZON_APPARENTLY))
print('Sunrise Sunset top of sun is apparently even with horizon:')
print(apparent_top_time.utc_iso(), apparent_top_up)
civil_twil_time, civil_twil_up = almanac.find_discrete(t0, t1,
daylength(planets, loc, DAYLENGTH_CIVIL_TWILIGHT))
print('Civil twilight:')
print(civil_twil_time.utc_iso(), civil_twil_up)
这将打印以下结果:
Sunrise Sunset center of sun is even with horizon:
['2019-02-03T14:20:33Z', '2019-02-04T00:05:20Z'] [ True False]Sunrise Sunset top of sun is apparently even with horizon:
['2019-02-03T14:15:28Z', '2019-02-04T00:10:25Z'] [ True False]Civil twilight:
['2019-02-03T13:44:36Z', '2019-02-04T00:41:18Z'] [ True False]
第一个列表显示发现更改的时间，第二个列表显示 True 如果太阳升起(或暮光开始)，False 当太阳升起时设置。
您的案例中缺少的 rough_period 应该是一个浮点值，表示一天发生的次数。太阳每天升起和落下一次，因此此函数中的事件每天发生两次。这意味着 rough_period 是 0.5。例如，当您想要计算月相时，可以将 rough_period 设置为 7.0(满月轨道为 27.3 天，每个月相为 6.825 天)。请参阅 almanac.py 中有关季节或月相计算的其他示例。源代码。
