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 myth of the swallowing of the Sun and the Moon by the demon snake.
According to legend, during the Samudra manthan, the asura Rahu drank some of the amrita (divine nectar). The sun and moon realized it and alerted Mohini (the female avatar of Vishnu). Mohini cut off the asura’s head before the nectar could pass his throat. The head, however, remained immortal due to the effect of amrita and became Rahu.
Rahu is seen as an asura or demon who does his best to plunge any area of life he controls into chaos, mystery, and cruelty. He is associated with the world of material manifestation and worldly desire, as well as random, uncontrolled growth without wisdom or understanding.
It is a legendary master of deception who signifies cheaters, pleasure seekers, operators in foreign lands, drug dealers, poison dealers, insincere and immoral acts. It is the symbol of an irreligious person, an outcast, harsh speech, falsehoods, uncleanliness, abdominal ulcers, bones, and transmigration. Sudden changes in luck and fame are also linked to Rahu. Rahu is considered instrumental in strengthening one’s power and converting even an enemy into a friend. It relates to the Seventh Ray energy of esoteric astrology, as it represents a force displaying all the possibilities within the realm of existence.
He is most powerful in Shatabhisha 6°40′ – 20° Aquarius (Kumbha) and also powerful in Ardra 6° 40′ – 20° Gemini (Mithuna) and Svati 6°40′ – 20° Libra (Tula). http://en.wikipedia.org/wiki/Rahu