The dasa results of a person depend upon the strength and position of planets and the strength of Yogas. Although there are several dasas like Kalachakradasa, Nissargadasa, Niryanadasa and Gulikadasa in astrology, the pride of place is given to Nakshatra dasa or star dasa.
Within the Nakshatra Dasa system, there are variations assigning different cycle periods and the most popular system is based on a cycle of 120 years, and this is called the Vimsottari Dasa System. In this system, human life is assigned a possible total of 120 years. These 120 years are divided into rulership of the 9 planets. Each planet has its own period of rulership and is called the dasa period of the planet.
The starting dasa of a person at the time of birth is determined based on the lord of the birth star. For example, a person born in either Aswini, Maka or Moola stars will have Kethu dasa at the beginning and those born in Bharani, Purva Phalguni or Poorvashada will have Venus dasa in the beginning, th eplanet being the lord of the birth star.
A person born at the beginning of Aswini star will have 7 years of Kethu Dasa. However, if at the time of birth some time has elapsed after the beginning of the period of the star, the balance of the dasa will be proportionately less. For example, if only 10 degrees of Ashwini star out of the total of 13 degrees 20 minutes is left, then we have to calculate the balance of the dasa at birth. The first dasa balance of a new born baby is called Sishtadasa or Garbhasishtadasa.