Dasa systems work very precisely. No astrological prediction is meaningful unless the same is synchronized with such a dasa system.

The unique feature of Indian System of astrological prediction is its quality of precision in revealing the periodic ups and downs in the course of an individuals life. The natal chart shows the potential. The order of events is described under Dasa and Mahadasa system. It is only on the basis of this system that the astrologer can indicate whether the inherent potential of any planet can actually fructify during the course of anyone's life.The ancient seers have indicated various planetary orders which bring different planets in prominence at different periods during the course of ones life. But they have not given the clues or the rationale for this order. They have assigned the course of these planets to be completed within certain number of years and within this total they have given different years of ruler ship to planets all of which are not equal. The reason of the total number of years of the cyclic duration, the sequence of planetary rulerships as well as the periods of their rulership is still a well guarded secret which is not known. Nonetheless, these Dasa systems work remarkably precisely and no astrological predictions is meaningful unless the same is synchronised with such a system.

Parashar has mentioned ten of these system as very important. They are known as Vimsottari, Ashtottari, Sodashottari, Dwasashottari, Panchottari, Shatshama, Chaturshityabda, Dwisaptatisama, Khastyabda and Sattrisadvatsara. Besides these, he also noted that there were several astrologers who also take into consideration Kala Dasa, Chakra Dasa and Kala Chakra Dasa. Parashara was also aware of several other systems of these planetary periodicity but he did not consider them very reliable......

The applicability of various Dasa systems in any specific individual depends upon time, place and the special planetary combinations present in the horoscope. The details of these factors are not generally available though some astrologers are found to apply a few of these Dasa Systems according to their individual prediction. Vimsottari Dasa, in spite of such a large number of Dasa Systems existing in astrological treatises, at present remains the most popular and universally acceptable system which has been giving very precise results.

The beginning of any planetary rulership in the case of an individual depends upon the Moons occupancy in a nakshatra, while the planetary rulership over different nakshatras begins with Sun as the ruler of Krittika and follows the same sequence throughout the nakshatra chain.

Vimsottari Dasa is based on the position of Moon at the time of birth in a specific nakshatra. The period of rulership of any planet at the time of birth depends upon the rulership of the planet on the nakshatra. The sequence of rulership of these planets follows a particular order, the rationale of which is still not known. This order is as follows: Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury, Ketu and Venus. The periodic rulership of the planets in any ones life, under this Dasa System also follows the same sequence. It implies that the beginning of any planetary rulership at the time of birth of an individual may be any one of these planets but once this is determined, the following periods of rulership must adhere to this sequence. That is, if the initial rulership period starts, say, with Saturn, then the next Dasa Period would be that of Mercury followed by Ketu and so on; if the initial rulership begins with say, Mars, then the following Dasa period would be that of Rahu, followed by Jupiter and so on.

The planetary sequence is the same in the case of planets rulership over different nakshatras. At birth under the Krittika nakshatra, that is, if the birth of the individual occurred when Moon was occupying Krittika nakshatra at the time of birth, the ruling planetary period would be that of Sun. Rohini follows Krittika, and any birth in Rohini nakshatra would have Moon as the ruling planet, and at Mrigashirsa that of Mars. The entire 27 nakshatras are divided in three rounds of planetary rulership each of these having the same sequence of planetary rulerships as indicated above. So the sequence of planetary rulership in the case of an individuals life as well as in the case of nakshatra rulership is the same: the beginning of any planetary rulership in the case of an individual depends upon the Moons occupancy in a nakshatra, while the planetary rulership over different nakshatras begins with Sun as the ruler of Krittika and follows the same sequence throughout the nakshatra chain.

The total planetary rulership under Vimsottari Dasa amounts to 120 year while in Ashtottari it is equal to only 108 years which shows that the total duration is not uniform for every dasa system.

The 120 years of total Vimsottari Dasa is allocated to different planets as follows : Sun 6 years; Moon 10 years; Mars 7 year; Rahu 18 years; Jupiter 16 years, Saturn 19 years; Mercury 17 years; Ketu 7 years; and Venus 20 years. These years of planetary rulerships according to some valid reason through the same is not know to us.

An important point to note is the way of calculating the initial balance of a planet at birth. It has been indicated above that the rulership of any planet starts according to Moons placement in nakshatra. Each nakshatra consists of 13° 20 of a sign of the zodiac. Aries starts with Ashwini nakshatra. It can, on the basis of information given above, be found out that this nakshtra is ruled by Ketu. It implies that a person born at the 0° of Aries which tantamount to 0° of Ashwini, and this portion of the sign of the zodiac is ruled by Ketu, will have the whole 7 years of Ketu at birth as balance to enjoy. The entire area of 0° 0 of Aries to 13° 20 of it which is under Ashwini is ruled by Ketu. But the individual may not be born at the very beginning of the asterism. The area already traversed and the remaining area yet to be covered by the Moon at birth will have to be taken into account for the determination of the period of Ketu yet to be enjoyed by the individual born. Assuming the individual was born when Moon was at 6° 40 of Aries that is, half-way through Ashwini, the Ketu balance at birth would be only 3 years and 6 months. Depending upon the distance yet to be covered by Moon in the asterism, the period has to be proportionately changed. Let us take another example. Assume that the Moon has gone further and is presently traversing Bharani which extends from 13° 20 of Aries to 26° 40 of the same sign. If there is another birth when Moon was at 16° 40 of this zodiac, then Venus would be the ruling planet and the distance between 13° 20 to 26 40 will have to be divided over 20 years which is the ruling period of Venus as against 7 years of Ketu. Moon at this new birth time has traversed only one fourth of the total distance, so only one fourth of its total rulership period has elapsed. This new born will have 15 years of Venus still to undergo after his or her birth.