Python: added documentation for function SearchRelativeLongitude.

source/python/README.md

@@ -722,6 +722,51 @@ dt_tolerance_seconds : float
 
 
 
+---
+
+<a name="SearchRelativeLongitude"></a>
+### SearchRelativeLongitude(body, targetRelLon, startTime)
+
+**Searches for when the Earth and another planet are separated by a certain ecliptic longitude.**
+
+Searches for the time when the Earth and another planet are separated by a specified angle
+in ecliptic longitude, as seen from the Sun.
+A relative longitude is the angle between two bodies measured in the plane of the
+Earth's orbit (the ecliptic plane). The distance of the bodies above or below the ecliptic
+plane is ignored. If you imagine the shadow of the body cast onto the ecliptic plane,
+and the angle measured around that plane from one body to the other in the direction
+the planets orbit the Sun, you will get an angle somewhere between 0 and 360 degrees.
+This is the relative longitude.
+Given a planet other than the Earth in `body` and a time to start the search in `startTime`,
+this function searches for the next time that the relative longitude measured from the
+planet to the Earth is `targetRelLon`.
+Certain astronomical events are defined in terms of relative longitude between
+the Earth and another planet:
+- When the relative longitude is 0 degrees, it means both planets are in the same
+  direction from the Sun. For planets that orbit closer to the Sun (Mercury and Venus),
+  this is known as *inferior conjunction*, a time when the other planet becomes very
+  difficult to see because of being lost in the Sun's glare.
+  (The only exception is in the rare event of a transit, when we see the silhouette
+  of the planet passing between the Earth and the Sun.)
+- When the relative longitude is 0 degrees and the other planet orbits farther from the Sun,
+  this is known as *opposition*. Opposition is when the planet is closest to the Earth,
+  and also when it is visible for most of the night, so it is considered the best time
+  to observe the planet.
+- When the relative longitude is 180 degrees, it means the other planet is on the opposite
+  side of the Sun from the Earth.  This is called *superior conjunction*.  Like inferior
+  conjunction, the planet is very difficult to see from the Earth.
+  Superior conjunction is possible for any planet other than the Earth.
+
+
+| Type | Parameter | Description |
+| --- | --- | --- |
+| [`Body`](#Body) | `body` | A planet other than the Earth. If `body` is not a planet, or if it is `Body.Earth`, an error occurs. |
+| `float` | `targetRelLon` | The desired relative longitude, expressed in degrees. Must be in the range [0, 360). |
+| [`Time`](#Time) | `startTime` | The date and time at which to begin the search. |
+
+
+
+
 ---
 
 <a name="SunPosition"></a>

source/python/astronomy.py

@@ -3760,6 +3760,56 @@ def _rlon_offset(body, time, direction, targetRelLon):
     return _LongitudeOffset(diff - targetRelLon)
 
 def SearchRelativeLongitude(body, targetRelLon, startTime):
+    """Searches for when the Earth and another planet are separated by a certain ecliptic longitude.
+
+    Searches for the time when the Earth and another planet are separated by a specified angle
+    in ecliptic longitude, as seen from the Sun.
+
+    A relative longitude is the angle between two bodies measured in the plane of the
+    Earth's orbit (the ecliptic plane). The distance of the bodies above or below the ecliptic
+    plane is ignored. If you imagine the shadow of the body cast onto the ecliptic plane,
+    and the angle measured around that plane from one body to the other in the direction
+    the planets orbit the Sun, you will get an angle somewhere between 0 and 360 degrees.
+    This is the relative longitude.
+
+    Given a planet other than the Earth in `body` and a time to start the search in `startTime`,
+    this function searches for the next time that the relative longitude measured from the
+    planet to the Earth is `targetRelLon`.
+
+    Certain astronomical events are defined in terms of relative longitude between
+    the Earth and another planet:
+
+    - When the relative longitude is 0 degrees, it means both planets are in the same
+      direction from the Sun. For planets that orbit closer to the Sun (Mercury and Venus),
+      this is known as *inferior conjunction*, a time when the other planet becomes very
+      difficult to see because of being lost in the Sun's glare.
+      (The only exception is in the rare event of a transit, when we see the silhouette
+      of the planet passing between the Earth and the Sun.)
+
+    - When the relative longitude is 0 degrees and the other planet orbits farther from the Sun,
+      this is known as *opposition*. Opposition is when the planet is closest to the Earth,
+      and also when it is visible for most of the night, so it is considered the best time
+      to observe the planet.
+
+    - When the relative longitude is 180 degrees, it means the other planet is on the opposite
+      side of the Sun from the Earth.  This is called *superior conjunction*.  Like inferior
+      conjunction, the planet is very difficult to see from the Earth.
+      Superior conjunction is possible for any planet other than the Earth.
+
+    Parameters
+    ----------
+    body : Body
+        A planet other than the Earth. If `body` is not a planet, or if it is `Body.Earth`, an error occurs.
+    targetRelLon : float
+        The desired relative longitude, expressed in degrees. Must be in the range [0, 360).
+    startTime : Time
+        The date and time at which to begin the search.
+
+    Returns
+    -------
+    Time
+        The date and time of the relative longitude event.
+    """
     if body == Body.Earth:
         raise EarthNotAllowedError()
     if body == Body.Moon or body == Body.Sun: