Introduction
Ketu is the tail or body of the Asura Svarbhanu, whose head is Rahu, and one of the nine celestial bodies (Navagraha) in Hindu astrology. He is the descending lunar node, the point opposite to Rahu, and is a "shadow planet" (chhaya graha). The word ketu means "banner" or "flag", and also "comet".
Origin
In the churning of the ocean of milk, when Vishnu cut off the head of Svarbhanu with his discus, the headless body fell and became Ketu. In some accounts, Ketu is treated as a separate being who arose from the severed body, and in some images he has a serpent's head.
Ketu and Rahu
Rahu and Ketu are always exactly 180 degrees apart on the ecliptic. They are described as the head and the tail of a single serpent-like being. In Indian astronomy, they move backwards (retrograde) among the stars, completing a cycle in about 18.6 years.
In astrology
Ketu represents detachment, spiritual liberation (moksha), isolation, past-life tendencies, and the loss of worldly things. While Rahu is worldly desire, Ketu is renunciation. Ketu is associated with sudden events, and also with the healing of wounds, and with mystic knowledge. In some traditions, Ketu is regarded as beneficial for spiritual development. He is associated with a smoky grey colour, and with the gem cat's eye (vaidurya).
Comets
The word ketu also means comet in classical Indian astronomy. Falling stars and comets were seen as omens and are described in texts such as the Brihat Samhita of Varahamihira.
Iconography
Ketu is depicted as a figure with the body of a human and the tail of a serpent, or as a headless figure carrying a mace, and riding a vulture.
Temples
The Keezhperumpallam temple in Tamil Nadu is the main temple of Ketu among the Navagraha temples. The Srikalahasti temple is known for Rahu-Ketu rituals.