确定合时坐标

今天金星的下合（虽然由于太阳偏北而仍然可以观测到）激发了 pyEphem 的以下研究。

1. 确定金星与太阳下合的日期。 pyEphem 中是否有金星和太阳之间（劣）合相的搜索功能？
2. 确定该日期金星的黄道纬度。那应该很容易。
3. 对过去 100 年和未来 100 年的合词执行此操作。这只是一个循环。

我想知道如何在 pyEphem 中做到这一点。

谢谢， 格特

在 PyEphem 中执行此操作会很尴尬，因为将其返回值减少为数字所需的操作 - 但它看起来类似于以下代码，它使用更现代的 Skyfield 库在 Python 中进行天文学。 （这段代码还需要安装SciPy，这样它的求解器就可以用来找到准确的合取时刻:)

import scipy.optimize
from skyfield.api import load, pi, tau

ts = load.timescale()
eph = load('de421.bsp')
sun = eph['sun']
earth = eph['earth']
venus = eph['venus']

# Every month from year 2000 to 2050.
t = ts.utc(2000, range(12 * 50))

# Where in the sky were Venus and the Sun on those dates?
e = earth.at(t)

lat, lon, distance = e.observe(sun).ecliptic_latlon()
sl = lon.radians

lat, lon, distance = e.observe(venus).ecliptic_latlon()
vl = lon.radians

# Where was Venus relative to the Sun?  Compute their difference in
# longitude, wrapping the value into the range [-pi, pi) to avoid
# the discontinuity when one or the other object reaches 360 degrees
# and flips back to 0 degrees.
relative_lon = (vl - sl + pi) % tau - pi

# Find where Venus passed from being ahead of the Sun to being behind.
conjunctions = (relative_lon >= 0)[:-1] & (relative_lon < 0)[1:]

# For each month that included a conjunction, ask SciPy exactly when
# the conjunction occurred.

def f(jd):
    "Compute how far away in longitude Venus and the Sun are."
    t = ts.tt(jd=jd)
    e = earth.at(t)
    lat, lon, distance = e.observe(sun).ecliptic_latlon()
    sl = lon.radians
    lat, lon, distance = e.observe(venus).ecliptic_latlon()
    vl = lon.radians
    relative_lon = (vl - sl + pi) % tau - pi
    return relative_lon

for i in conjunctions.nonzero()[0]:
    t0 = t[i]
    t1 = t[i + 1]
    print("Starting search at", t0.utc_jpl())
    jd_conjunction = scipy.optimize.brentq(f, t[i].tt, t[i+1].tt)
    print("Found conjunction:", ts.tt(jd=jd_conjunction).utc_jpl())
    print()

无论您使用哪个天文学库，这都是要采取的一般方法：随着时间向前迈出一大步（在本例中为 1 个月的步长），寻找金星在经度上从太阳前面移动到太阳后面的时间。然后，返回每个月份，并在经度相同的确切时刻归零。以下是上述脚本打印的一些值，您可以根据 USNO 或其他来源发布的值进行抽查：

Starting search at A.D. 2013-Dec-24 00:00:00.0000 UT
Found conjunction: A.D. 2014-Jan-11 12:24:30.8031 UT

Starting search at A.D. 2015-Jul-28 00:00:00.0000 UT
Found conjunction: A.D. 2015-Aug-15 19:21:55.1672 UT

Starting search at A.D. 2017-Mar-01 00:00:00.0000 UT
Found conjunction: A.D. 2017-Mar-25 10:17:27.5276 UT

Starting search at A.D. 2018-Oct-03 00:00:00.0000 UT
Found conjunction: A.D. 2018-Oct-26 14:16:19.3941 UT

Starting search at A.D. 2020-May-06 00:00:00.0000 UT
Found conjunction: A.D. 2020-Jun-03 17:43:37.7391 UT