如何从pyephem中获取太阳经度？

Question

我正在尝试确定火星上季节的开始和结束时间。这个时间点是在0º、90º、180º和270º的太阳经度。

PyEphem提供了日心坐标的经度和纬度。请问这和这里显示的太阳经度有什么关系呢：

http://www.planetary.org/explore/space-topics/mars/mars-calendar.html

Answer 1

我觉得这两个量是相同的角度，只是从不同的起点来测量。基于这个假设，你可以用PyEphem来预测火星的季节，误差大概在两到三天之内——不过我本来期待能更准确一点，不太明白为什么结果没有那么接近！

不过这至少可以让你入门。如果你发现这些预测和你链接的那个网站的结果有些不同，记得告诉我们！我们应该找个时间把它们和NASA Horizons的结果，以及我正在开发的Skyfield项目的结果进行比较，Skyfield是一个改进版的PyEphem。

无论如何，这里有一些代码可以用来计算Ls，并用它来找出火星的季节：

# The angle that we call "the longitude of the Sun, as
# seen from Mars" should grow at the same rate as the
# "longitude of Mars as seen from the Sun" (since the
# two are the same line but viewed in opposite
# directions).
#
# The only problem is knowing what point to name "zero",
# so we have to learn what .hlon was when the first
# Martian year started:

from ephem import Mars, Date, degrees, newton
m = Mars()
m.compute('1955/4/11 23:00')
Ls0 = m.hlon

def Ls(date):
    m.compute(date)
    return degrees(m.hlon - Ls0).norm

# There!  Ls() should give Martian solar latitude.
# So the first round of seasons listed at the page
# http://www.planetary.org/explore/space-topics/mars/mars-calendar.html
# should give 90 degrees, 180 degrees, and 270 degrees:

for date in '1955/10/27', '1956/4/27', '1956/9/21':
    print Ls(date)

# The output is close to what we would expect:
#
# 90:11:58.3
# 179:57:32.2
# 270:13:22.6
#
# Great!  So what if we want to know, say, the date
# of the upcoming Spring Equinox or Summer Solstice?
# We need functions that are smooth, well-behaved,
# and cross zero at those two times, so that we can
# unleash Newton's Method upon them:

def spring_equinox(date):
    return Ls(date).znorm

def summer_solstice(date):
    return Ls(date) - degrees('90:00:00')

def find_spring_equinox(start_date):
    start_date = Date(start_date)
    y0 = Ls(start_date)
    y1 = Ls(start_date + 1)
    rate = y1 - y0
    angle_to_go = degrees(0.0 - y0).norm
    closer_date = start_date + angle_to_go / rate
    d = newton(spring_equinox, closer_date, closer_date + 1)
    return Date(d)

def find_summer_solstice(start_date):
    start_date = Date(start_date)
    y0 = Ls(start_date)
    y1 = Ls(start_date + 1)
    rate = y1 - y0
    angle_to_go = degrees(degrees('90:00:00') - y0).norm
    closer_date = start_date + angle_to_go / rate
    d = newton(summer_solstice, closer_date, closer_date + 1)
    return Date(d)

d = find_spring_equinox('2015/1/22')
print d, Ls(d)

d = find_summer_solstice('2015/1/22')
print d, Ls(d)

# Output:
# 2015/6/16 15:03:15 0:00:00.0
# 2015/12/31 21:12:07 90:00:00.0