Rahu is the ascending node and Ketu is the descending node of Moon.They are called as the Dragon's head and the Dragon's tail in Western astrology.Rahu is a shadowy planet worshipped as a deityin Indian astrology. Rahu is said to rule all dark forces and black practices.Rahu is the planet that rules the activities of sudden losses or sudden profits.
Ketu is known to afflict the Sun more than any other planet. This is in contrast to Rahu, which is said to afflict the Moon. Both Rahu and Ketu have no substance and are physically non-existent. Yet their influence is full of potency and spiritual significance. Their transits play an important role in our lives.
A full transit cycle of Rahu-Ketu lasts 18 years. Rahu-Ketu transit through each rashi axis for a period of 18 months. Rahu returns to each rashi every 18 years, and remains in that rashi for 18 months.