Ketu

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

A graha is not read in isolation but through the several tables that make up a Panchanga and a chart. It rules no weekday, because the seven-day week is built from the seven visible grahas and the two nodes stand outside that set. It holds 7 years of the Vimshottari dasha cycle, and it is lord of three of the twenty-seven nakshatras, Ashwini, Magha and Mula, spaced nine apart so that the nine grahas cover the twenty-seven exactly three times over.

The Vimshottari dasha system divides a notional lifetime of 120 years among the nine grahas in a fixed order, Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn and Mercury, with fixed lengths that sum to exactly 120. Which period a person begins life in is determined by the nakshatra the Moon occupied at birth, and how far through that period, by how far through the nakshatra the Moon had travelled.

In the scheme of dignities it is assigned an exaltation on which the classical sources do not agree, and dignity is what the classical rules for strength are built on, since a graha in its own sign or its exaltation is read as able to act and one in its fall as constrained.

Ketu is the descending node, the opposite point to Rahu and therefore always exactly a hundred and eighty degrees from it in any chart. The two are computed rather than observed, they always move retrograde through the signs, and neither has a weekday, since the seven-day week is built on the seven visible grahas and the nodes are additions to that older set.