Fix #361 by adding a lunar node almanac routine

CHANGELOG.rst

@@ -9,9 +9,12 @@ Changelog
 * To hopefully fix the ``SSL: CERTIFICATE_VERIFY_FAILED`` errors that
   some users encounter when downloading timescale files, took the risk
   of switching away from Python’s default SSL certificates to the
-  certificate bundle in the `certifi` package.
+  certificate bundle in the ``certifi`` package.
   `#317 <https://github.com/skyfielders/python-skyfield/issues/317>`_
 
+* Added a new almanac routine for finding :ref:`lunar-nodes`.
+  `#361 <https://github.com/skyfielders/python-skyfield/issues/361>`_
+
 * Gave topos objects a new :meth:`~skyfield.toposlib.Topos.itrf_xyz()`
   method that returns their raw ITRF coordinates.
   `#354 <https://github.com/skyfielders/python-skyfield/issues/354>`_

skyfield/almanac.py

@@ -120,6 +120,31 @@ def moon_phase_at(t):
     moon_phase_at.rough_period = 7.0  # one lunar phase per week
     return moon_phase_at
 
+MOON_NODES = [
+    'descending',
+    'ascending',
+]
+
+def moon_nodes(ephemeris):
+    """Build a function of time that identifies lunar nodes.
+
+    This returns a function taking a :class:`~skyfield.timelib.Time` and
+    returning ``True`` if the Moon is above the ecliptic else ``False``.
+    See :ref:`lunar-nodes` for how to use this routine.
+
+    """
+    earth = ephemeris['earth']
+    moon = ephemeris['moon']
+
+    def moon_node_at(t):
+        """Return the phase of the moon 0 through 3 at time `t`."""
+        e = earth.at(t)
+        lat, _, _ = e.observe(moon).apparent().ecliptic_latlon('date')
+        return lat.radians > 0.0
+
+    moon_node_at.rough_period = 14.0  # one node each half lunar month
+    return moon_node_at
+
 CONJUNCTIONS = [
     'conjunction',
     'opposition',

skyfield/documentation/almanac.rst

@@ -129,6 +129,31 @@ corresponding array of Moon phases with 0 for New Moon and 3 for Last
 Quarter.  You can use the array ``MOON_PHASES`` to retrieve names for
 each phase.
 
+.. _lunar-nodes:
+
+Lunar Nodes
+===========
+
+The Moon’s ascending node and descending node are the moments each lunar
+month when the Moon crosses the plane of Earth’s orbit and eclipses are
+possible.
+
+.. testcode::
+
+    t0 = ts.utc(2020, 4, 22)
+    t1 = ts.utc(2020, 5, 22)
+    t, y = almanac.find_discrete(t0, t1, almanac.moon_nodes(e))
+
+    print(t.utc_iso())
+    print(y)
+    print([almanac.MOON_NODES[yi] for yi in y])
+
+.. testoutput::
+
+    ['2020-04-27T17:54:17Z', '2020-05-10T09:01:42Z']
+    [ True False]
+    ['ascending', 'descending']
+
 .. _oppositions-conjunctions:
 
 Opposition and Conjunction

skyfield/documentation/api-almanac.rst

@@ -18,6 +18,8 @@ times of sunrise, sunset, and the phases of the moon.
 
 .. autofunction:: moon_phases
 
+.. autofunction:: moon_nodes
+
 .. autofunction:: oppositions_conjunctions
 
 .. autofunction:: sunrise_sunset