Rahu

In Hindu tradition, Rahu is a cut-off head of an asura, that swallows the sun causing eclipses. He is depicted in art as a serpent with no body riding a chariot drawn by eight black horses. Rahu is one of the navagrahas (nine planets) in Vedic astrology. The Rahu kala (time of day under the influence of Rahu) is considered inauspicious.

According to legend, during the Samudra manthan, the asura Rahu drank some of the amrita (divine nectar). Sun and moon realized it and they alerted Mohini (the female avatar of Vishnu). Before the nectar could pass his throat, Mohini cut off his head. The head, however, remained immortal due to the effect of amrita and became Rahu. It is believed that this immortal head from time to time swallows the sun, causing eclipses. Then, the sun passes through the opening at the neck, ending the eclipse. The body also turned into Ketu due to a boon, and it in turn swallows the moon on timely basis to cause a lunar eclipse.

Astronomically, Moon in its orbit, on a northerly course from South to North, crosses the ecliptic (the apparent path of Sun). This point of intersection is known as Rahu or Dragons head. 180 degrees away from this point, Moon on its southerly course crosses the ecliptic. This point is called the dragon’s tail or Ketu.

Thus, Ketu and Rahu denote the points of intersection of the paths of the Sun and the Moon as they move around 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 is explained in stories as the swallowing of the Sun and the Moon by the demon snake.

Thus, Rahu and Ketu do not have physical identity but only mathematically calculated sensitive points. The Rahu and Ketu points are not stationary in space but have a mean motion of about 19 degrees and 30 minutes in a year. It means it takes about 18 years and 6 months to make a revolution around the Earth. This motion is retrograde. There is a concept of Mean Rahu and True Rahu. The ancient Hindu astrology takes the true position of Rahu and Ketu. It is unique in the sense that it was the first to have a dedicated name for these identified astronomical points that are helpful in deducing eclipses.

Rahu is a legendary master of deception who signifies cheaters, pleasure seekers, operators in foreign lands, drug dealers, poison dealers, insincere & immoral acts, etc. It is the significator of an irreligious person, an outcast, harsh speech, falsehoods, uncleanliness, abdominal ulcers, bones, and transmigration. Rahu is instrumental in strengthening one's power and converting even an enemy into a friend. In Buddhism Rahu is one of the krodhadevatas (lit: Anger divinities. i.e., a divinity having the attributes of anger and the like).

Read more about Rahu:  Astrology, Worship of Rahu, Remedies, Rahu Mantra, Hanuman and Rahu, Buddhism