Nakshatras are not the basis of all dasa systems. There are several dasa systems that have nothing to do with nakshatras. However, the most popular systems are nakshatra based. And the most popular of all, vimshottari dasa, is among the nakshatra systems.
The reason nakshatras are essential in calculating the flow of time can be known by knowing the first part of the first chapter of Surya Siddhanta. There it explains:
There are two types of time: philosophical and practical. The Surya Siddhanta concerns itself with practical time. There are four types of practical time systems: sidereal, tropical, lunar, and civil.
Civil time is counted by sunrises. Each sunrise is a day. 30 sunrises is a month, and 12 such months is a civil year. It is the most simple way of reckoning practical time.
Lunar time is measured by the relative distance between the Sun and Moon. So it is a type of sidereal time (that is, it has no reference to the earth, only to the heavenly bodies). Each day of lunar time is a certain angle of distance between the Sun and Moon, called a “tithi.” 30 tithis is a month, and 12 such months is a lunar year.
Tropical time is measured by the relationship between the Sun and the Earth, specifically between the Sun and the Earth’s equator. This relationship defines the equinoxes, solstices and seasons. 1 degree of motion relative to the spring equinox constitutes one tropical day. Again 30 of them is a tropical month, and 12 of those is a tropical year.
Sidereal time in the Surya Siddhanta is not the same as modern sidereal time. Modern sidereal time is based on a sidereal year. Surya Siddhanta’s sidereal time is based on a sidereal day. A sidereal day is the amount of time elapsed between the rising of a given star (a star, not the Sun) in the east. Like always, 30 such days is a sidereal month, and 360 of them is a sidereal year by Surya Siddhanta’s calculations.
After giving these definitions, the Surya Siddhanta goes on to define three of the four as having specific limited application. Tropical time is specifically for determining rashis (the 12 “signs”) and seasons – which is most relevant to farming and other practical deeds. Lunar time is specifically for determine sacred events. Civil time is specifically meant for cases when long term accuracy is not important.
Sidereal time is therefore the primary way for counting the unfolding of practical time. “Philosophical time” is the conceptual force which causes creation and destruction. “Practical time” is the changing of one moment into the next so that events can unfold. So the Surya Siddhanta defines sidereal time as the primary measure for normal time – the unfolding of events.
“Sidereal” in sanskrit is “Nakshatra.” Nakshatra dasa in English literally means “sidereal phase.” A nakshatra dasa is a phase of sidereal time. Therefore the nakshatra (27 stars) of the sidereal zodiac are the main milestones measuring the pace of this type of time.
The most popular astrological timing systems in India are based on sidereal nakshatra because the whole purpose of a timing system is to understand how events will unfold in due course, and the correct way of measuring that is nakshatra-based time.
It begs mentioning that surya-siddhanta’s nakshatra year is the equivalent of 359.017 of our modern calendar days. So this is the length of the year that should be used when calculating the duration of nakshatra dasas.
– Vic DiCara