python - pyephem 的太阳经度？

Question

我正在尝试确定火星上季节的开始和结束时间。该点位于 0º、90º、180º 和 270º Ls(太阳经度)。

PyEphem 提供日心经度和纬度。这与此处显示的太阳经度有何关联:

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

Answer 1

我认为这两个量是同一个角度，但是从不同的起点测量的。使用该假设，您可以使用 PyEphem 在两三天内预测火星季节——但我真的期望更好，但我不确定为什么协议(protocol)没有更接近!

但这至少会让您入门。如果您发现这些预测与您链接到的网站的预测略有不同，请告诉我们!我们应该在某个时候将它们与 NASA Horizo​​ns 的输出以及我作为改进的 PyEphem 构建的 Skyfield 项目的输出进行比较。

无论如何，一些代码可以计算 Ls 并用它来查找火星季节:

# 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