Rahu

Rahu is the eighth graha, the Moon's ascending node, not a body but a calculated point where the Moon's path crosses the ecliptic, where eclipses occur. It is one of the two chhaya grahas, the shadow planets. Its presiding deity is Durga, its gemstone is hessonite (gomed), its element is air, and its Vimshottari period is eighteen years. In myth Rahu is the severed head of a demon who stole the nectar of immortality and now swallows the Sun and Moon at eclipses. In jyotisha Rahu signifies worldly desire, obsession, foreignness, illusion, and sudden amplification, it magnifies whatever it touches and is classed as a malefic. It has no rulership of its own but takes on the character of its sign lord and dispositor. These are the tradition's significations for the node; the underlying reality is the geometry of the lunar orbit that also governs eclipse seasons.

Rahu's presence among the grahas illustrates the class's Sanskrit definition especially clearly: graha comes from the root grah-, to seize or grasp, and the nine grahas are properly seizers of a nativity rather than planets in the astronomical sense, a definition that lets Rahu, no body at all but a calculated point where the Moon's path crosses the ecliptic, take a full place beside the Sun and Moon. The ninefold navagraha scheme that includes Rahu and Ketu is itself an Indian development rather than a Greek inheritance, datable to roughly the third through eighth centuries CE; Greek horoscopy, which reached Sanskrit through the Yavanajataka of the second to fourth centuries CE and supplied the machinery of dignity and exaltation for the other seven grahas, worked with only seven bodies and had no equivalent doctrine of the lunar nodes as agents with significations of their own.

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 18 years of the Vimshottari dasha cycle, and it is lord of three of the twenty-seven nakshatras, Ardra, Swati and Shatabhisha, 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.

Rahu is not a body. It is the ascending node of the Moon's orbit, the point where the Moon crosses the plane of the ecliptic going north, and it is one of the two points at which eclipses can occur. That is why the tradition calls the nodes the shadow grahas and tells the story of a severed demon who swallows the Sun and Moon: the mythology and the eclipse geometry are describing the same thing.