The Rahu Mahadasha is the period of the north node in the Vimshottari dasha, lasting eighteen years, the second longest of the nine. In the fixed sequence it follows Mars and precedes Jupiter. As the shadow graha of worldly desire, obsession, foreignness, and sudden amplification, Rahu in its dasha is traditionally read as a long and often turbulent season of ambition, unconventional pursuits, rapid rises and reversals, and engagement with the unfamiliar, its results shaped by Rahu's placement and its dispositor in the birth chart. The dasha a person begins life in is fixed by the nakshatra of the natal Moon. These are the interpretive conventions of the tradition; the Vimshottari system is a timing method keyed to the Moon's position at birth.
Rahu's eighteen years is the second longest mahadasha after Venus's twenty, and classical Jyotisha treats the lunar nodes, Rahu and Ketu, as chhaya grahas, shadow points rather than physical bodies, calculated from the intersections of the Moon's orbit with the ecliptic. Because a shadow point owns no physical sign in the way the seven visible grahas do, classical authorities have never fully agreed on Rahu's own sign, exaltation and mulatrikona assignments, with different texts and different regional schools of Jyotisha giving somewhat different tables; this disagreement over Rahu's essential dignity is a separate and older dispute from the Vimshottari period length itself, which all systems treat as fixed at eighteen years regardless of which dignity table is followed.