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