在太阳上使用哪个场？
该
sun.alt是正确的。
alt是地平线以上的高度；它们与北方以东的方位角一起确定了相对于地平线的视在位置。
您的计算几乎是正确的。您忘记提供观察员了：
sun = ephem.Sun(o)。
- 我不知道如何解释cot（phi）的负面结果。有人能帮我吗？
在这种情况下，太阳在地平线以下。
最后，对于给定ephem.Observer（），我对如何使用PyEphem从阴影长度向后工作感到困惑，直到下次太阳将投射该长度的阴影时。
这是一个提供功能的脚本：
g(date) -> altitude计算下次太阳投射阴影的时间与当前长度相同（方位角-不考虑阴影的方向）：
#!/usr/bin/env pythonimport mathimport ephem import matplotlib.pyplot as pltimport numpy as npimport scipy.optimize as optdef 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 timezonedef 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 inplacedef 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 methodif __name__=="__main__": main()
输出量
current time: 2011/4/19 23:22:52 next time: 2011/4/20 13:20:01
