Rāhu (Sanskrit: राहु)() is one of the nine major astronomical bodies (navagraha) in Indian texts. Unlike the other eight, Rāhu is not a real astronomical body but a shadow entity, one that causes eclipses and is the king of meteors.[1] Rahu represents the ascend of the moon in its precessional orbit around the earth.
Rāhu is usually paired with Ketu. The time of day considered to be under the influence of Rāhu is called Rāhu kāla and is considered inauspicious.[2]
As per Vedic astrology Rahu and Ketu have an orbital cycle of 18 years and are always 180 degrees from each other orbitally (as well as in the birth charts). 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. This also corresponds to a saros, a period of approximately 223 synodic months (approximately 6585.3211 days, or 18 years, 11 days, 8 hours), that can be used to predict eclipses of the Sun and Moon.
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. The fact that eclipses occur when the Sun and the Moon are at one of these points gives rise to the understanding of swallowing of the Sun and the Moon by the snake. (wiki)
