Ketu is the ninth graha, the Moon's descending node, the point opposite Rahu where the lunar path crosses the ecliptic on the way down. It is the second of the shadow planets. Its presiding deity is Ganesha, its gemstone is cat's-eye (lehsunia), its element is fire, and its Vimshottari period is seven years. In myth Ketu is the tail of the same demon whose head is Rahu, severed by Vishnu's discus. In jyotisha Ketu signifies detachment, loss, spirituality, past-life residue, and liberation (moksha), where Rahu grasps outward, Ketu lets go, and it is read as the planet of what the soul has already mastered and now transcends. It is classed as a malefic but is also the great significator of spiritual release. Like Rahu it has no sign of its own and acts through its dispositor. These are jyotisha's significations for the descending node of the Moon's orbit.
Ketu's standing as a graha with no physical body makes the class's Sanskrit definition unusually explicit: graha is from the root grah-, to seize or grasp, and the nine grahas are seizers of a nativity by that grip rather than planets in the astronomical sense, a definition broad enough to include Ketu, a calculated point where the Moon's path crosses the ecliptic on its descending node. The ninefold navagraha scheme that gives Rahu and Ketu a place alongside the seven classical bodies is an Indian development, not inherited from Greek sources, generally dated to roughly the third through eighth centuries CE; the Yavanajataka, the Sanskrit versification of a Greek astrological text from the second to fourth centuries CE that supplied the dignities and exaltations of the other seven grahas, reflects a Greek horoscopic tradition that worked with only seven bodies and assigned the nodes no comparable significations.
Sources Brihat Parashara Hora ShastraSage Parashara (attributed) with Jyotihsastra: Astral and Mathematical LiteratureDavid Pingree