 This coincides with the precessional orbit of moon or the ~18-year rotational cycle of the lunar ascending and descending nodes on the earth's ecliptic plane.Ketu rules the Scorpio zodiac sign together with Mangala

Astronomically, Rahu and Ketu denote the points of intersection of the paths of the Sun and the Moon as they move on the celestial sphere.Therefore, Rahu and Ketu are respectively called the north and the south lunar nodes.