Add possibility to calculate Moon South Node position

src/gswe-moment.c

@@ -1012,6 +1012,12 @@ gswe_moment_calculate_planet(GsweMoment *moment,
             );
     }
 
+    // The south node is actually on the opposite side of the chart,
+    // so let’s invert the position.
+    if (planet == GSWE_PLANET_MOON_SOUTH_NODE) {
+        x2[0] = fmod(x2[0] + 180.0, 180.0);
+    }
+
     calculate_data_by_position(moment, planet, x2[0], &calc_err);
 
     if (calc_err != NULL) {

src/gswe-types.h

@@ -39,6 +39,7 @@
  * @GSWE_PLANET_VERTEX: the Vertex (the point where the ecliptic meats the
  *                      primal vertical)
  * @GSWE_PLANET_MOON_NODE: the mean ascending (north) Moon node
+ * @GSWE_PLANET_MOON_SOUTH_NODE: the mean descending (south) Moon node
  * @GSWE_PLANET_MOON_APOGEE: the mean Moon apogee (sometimes called Dark Moon,
  *                           or Lilith)
  * @GSWE_PLANET_SUN: the Sun
@@ -78,6 +79,7 @@ typedef enum {
     GSWE_PLANET_VERTEX,
     GSWE_PLANET_MOON_NODE,
     GSWE_PLANET_MOON_APOGEE,
+    GSWE_PLANET_MOON_SOUTH_NODE,
 
     /* Actual astrological planets */
     GSWE_PLANET_SUN = 11,

src/swe-glib.c

@@ -202,6 +202,13 @@ void gswe_init_with_dir(gchar *directory)
             2.0,
             1
         );
+    ADD_PLANET(gswe_planet_info_table, planet_info,
+               GSWE_PLANET_MOON_SOUTH_NODE,
+               SE_MEAN_NODE,
+               TRUE,
+               _("Descending Moon Node"),
+               2.0,
+               0);
     ADD_PLANET(gswe_planet_info_table, planet_info,
             GSWE_PLANET_MOON_APOGEE,
             SE_MEAN_APOG,