In Hindu astrology, Antardasha is the sub-period assigned to each of the planets, during their mahadasha (main period). The dasa system, a feature of Hindu astrology, is based on the premise that the planets in a unique cyclic order generate the good as well as bad effects during the life of all human beings.
Vimsottaraidasa, the life-span of 120 years, has been divided in the following order; the main period of 6 years for the Sun, 10 for the Moon, 7 for Mars, 18 for Rahu, 16 for Jupiter, 19 for Saturn, 17 for Mercury, 7 for Ketu and 20 for Venus. However, the initial phase, different for different people, is decided by the nakshatra (asterism), occupied by the Moon at the time of birth. All the 27 asterisms are associated with planets.
For instance, the asterisms, Krittika, Uttaraphalguni, and Uttarashada, are assigned to the Sun and the rest follow the order given above. Suppose the Moon is transiting through the Chitra asterism at birth, then the mahadasha would start with Mars, whose duration is calculated in proportion to the uncrossed part of the Chitra. The planets, next in the cyclic order, would have their full main periods. Each period of mahadasha is further divided into nine antardashas (sub-periods), which are calculated in the ratio of their main periods.
The initial phase of the antardasha is assigned to the planet whose mahadasha is running. Other planets then follow the cyclic order. For instance, in the mahadasha of Venus, the first antardasha would be of Venus itself, for 40 months. The antardasha of the Sun would follow for one year, and so on.