The Question of Ayanaṁśa
The first question is “exactly where is the anchor of the nakshatras, and thus exactly where are their borders?” This question affects the length of daśās because an individual is born at a particular spot in the flow of the daśas based on the location of the moon relative to the borders of the nakshatras at the time of his or her birth.
The “ayanamsha” is what defines the anchor (thus the borders) of the nakṣatras. Let’s use me for an example. The most standard, widely used ayanāṁśa is named after the person who defined it: Lahiri. According to it, I was born when the moon was 41 3/4 degrees from the anchor point. The most widely used definition of Nakṣatras is that each one spans 13 1/3 degrees. So the fourth nakṣatra, Rohiṇī, begins at 13 1/3 X 3 = 40 degrees and ends at 53 1/3 degrees. According to Laihi ayanaṁśa, therefore, I was born with the moon 1 3/4 degrees into Rohiṇī.
Since the Rohinī nakṣatra corresponds to the Moon daśā (in the most widely used viṁśottarī system), I was born in the Moon daśā. Since the moon wasn’t exactly at the very beginning of Rohiṇī, I wasn’t born at the very beginning of the Moon daśā. I was born when the moon was 1.75 degrees into Rohiṇī. How far is that? Well its 1 3/4 / 13 1/3 = 13% of the way through Rohiṇī. Therefore I was born 13% of the way through the Moon daśā. Which translates to: “Moon / Mars / Venus / Saturn / Moon” at five levels of daśā specification.
Now change the ayanamśa to something else, and see what happens.
Let’s define the stars the way Fagan/Bradly do, instead of Lahiri. By their calculations I was born when the Moon was at 40 degrees and 50 minutes from the anchor point. That still puts it in the 4th nakṣatra, Rohiṇī. But only by 50 minutes. That means, by their definitions, I was born when the moon was only 6% of the way through Rohiṇi, and thus my daśās start 6% of the way into the moon daśā, which translates to: “Moon / Moon / Ketu / Mercury / Rahu.”
Since the daśā starting point changes when the ayanamsha changes, the timings of all the daśās shift relative to the starting point. By Fagan/Bradley’s version Rahu daśā began for me when I was 16 years and a little less than one month old, on August 22, 1986. By Lahiri’s definitions, Rahu daśā began when I was about 15 and a half – about half a year earlier, on December 27, 1985.
Since Lahiri calculates my natal moon to be further into Rohini, I start my life later in the Moon daśā, which therefore finishes and changes to the next daśa sooner than Fagan Bradley – who calculate my natal moon to be just barely into Rohini.
There are many different opinions on ayanaṁśa. Kala software lists 12 different options and also allows you to use no ayanamśa, or to define your own. Obviously this is an “X” in the equation which causes mathematical, fundamental uncertainty in our systems.
The Question of Varṣa
Varṣa means “year.” The astrologers of yore who defined daśā systems always did so using the year as their unit of measure. But there are at least four different ways to calculate the length of the year – and the astrologers of yore knew that too (they are defined in Sūrya Siddhānta, for example). I have explained this in more detail before ( http://wp.me/pHPQr-o0 ).
What it boils down to is that when the daśā system we are using is sidereal we have to use a sidereal year. All nakṣatra based daśā are sidereal. (There are other daṣā system that are raśi based, and therefore might arguably use the tropical year). Even then there are two types of sidereal years. One is the sidereal movement of the sun, and the other is purely sidereal (see link from last paragraph for more explanation). Since nakṣatra daśā have nothing to do with the sun, we should probably use the purely sidereal year. Furthermore, the Sūrya Siddhanta defines what the different types of years pertain to – and it appears from those definitions that the purely nakshatra year is used for astrological math (again, see the previous link for more explanation).
However the vast majority of contemporary astrologers and would-be-astrologers use a modern tropical calendar year for all these calculations. The difference is that the tropical year is about 365 days while the purely sidereal year is about 359 days (“days” = sunrises). So there is a discrepancy.
The discrepancy is small, only 6 days per year, but in 10 years thats 60 days discrepancy. In 100 years it’s 600 days, about two whole years. So actually the question of properly defining the year is more consequential or at least similarly consequential as defining the ayanāṁśa accurately.
Personally – I use Lahiri’s definition of the nakṣatras because his definition makes logical astronomical sense and because in my practice I have yet to come to a situation that demands I serious question it. However, I do not use the tropical year for nakṣatra daśā calculations. It is not logical, and I did come into situations in practice that demanded I seriously question it.