在太阳上使用什么场？
sun.alt 是正确的。 alt 是地平线以上的高度；连同北以东的方位角，它们定义了相对于地平线的apparent position。
您的计算几乎是正确的。你忘了提供观察者：sun = ephem.Sun(o)。
- 我不知道如何解释 cot(phi) 的阴性结果。能
有人帮我吗？
在这种情况下，太阳在地平线以下。
最后，我对如何使用感到困惑
PyEphem 从
影子长度到下次什么时候
太阳会投下那个阴影
长度，给定一个 ehem.Observer()。
这是一个给定函数的脚本：g(date) -> altitude 计算下一次太阳投射与现在相同长度的阴影的时间（不考虑方位角 -- 阴影的方向）：
#!/usr/bin/env python
import math
import ephem
import matplotlib.pyplot as plt
import numpy as np
import scipy.optimize as opt
def main():
# find a shadow length for a unit-length stick
o = ephem.Observer()
o.lat, o.long = '37.0625', '-95.677068'
now = o.date
sun = ephem.Sun(o) #NOTE: use observer; it provides coordinates and time
A = sun.alt
shadow_len = 1 / math.tan(A)
# find the next time when the sun will cast a shadow of the same length
t = ephem.Date(find_next_time(shadow_len, o, sun))
print "current time:", now, "next time:", t # UTC time
####print ephem.localtime(t) # print "next time" in a local timezone
def update(time, sun, observer):
"""Update Sun and observer using given `time`."""
observer.date = time
sun.compute(observer) # computes `sun.alt` implicitly.
# return nothing to remember that it modifies objects inplace
def find_next_time(shadow_len, observer, sun, dt=1e-3):
"""Solve `sun_altitude(time) = known_altitude` equation w.r.t. time."""
def f(t):
"""Convert the equation to `f(t) = 0` form for the Brent's method.
where f(t) = sun_altitude(t) - known_altitude
"""
A = math.atan(1./shadow_len) # len -> altitude
update(t, sun, observer)
return sun.alt - A
# find a, b such as f(a), f(b) have opposite signs
now = observer.date # time in days
x = np.arange(now, now + 1, dt) # consider 1 day
plt.plot(x, map(f, x))
plt.grid(True)
####plt.show()
# use a, b from the plot (uncomment previous line to see it)
a, b = now+0.2, now+0.8
return opt.brentq(f, a, b) # solve f(t) = 0 equation using Brent's method
if __name__=="__main__":
main()
输出
current time: 2011/4/19 23:22:52 next time: 2011/4/20 13:20:01