The method of calculating Naisargika Dasa is described in Varaha Mihira’s "Brihat Jataka", chapter 8 sloka 9:
"The Naisargika (natural) dasas in case of all creatures are those of the Moon, Mars, Mercury, Venus, Jupiter, the Sun and Saturn in the order stated, and their periods are respectively 1, 2, 9, 20, 18, 20 and 50 years. If the Naisargika Dasa period and the ordinary planetary Dasa [Moola Dasa?] happen to run together, such period will be a prosperous one. According to Yavanesware, the closing period is the Naisargika Lagna Dasa and produces prosperity. This is objected by some".
I have seen no evidence that Varaha Mihira ever used Vimsottari Dasa, but it seems that Varaha Mihira used Naisargika Dasa side by side with Moola Dasa, perhaps to get a second opinion.
I have also used it like that, but since the descriptions on how to calculate the Moola Dasa seems to be somewhat obscure and difficult to follow, I have used Naisargika Dasa side by side with Vimsottari Dasa.
I have not come accross any mentioning of sub-periods in connection to Naisargika Dasa. For Moola Dasa however each Dasa seems to be divided into 4 different sub-periods.
Anyway, based on the information in the sloka given by Varaha Mihira, the Naisargika Dasa is very easy and fast to calculate, since the dasa-periods are the same for all human beings. There are only seven dasa-periods, and they run like this:
1st dasa: Moon Period: 0- 1 years Duration: 1 year
2nd dasa: Mars Period: 1- 3 years Duration: 2 years
3rd dasa: Mercury Period: 3- 12 years Duration: 9 years
4th dasa: Venus Period: 12- 32 years Duration: 20 years
5th dasa: Jupiter Period: 32- 50 years Duration: 18 years
6th dasa: Sun Period: 50- 70 years Duration: 20 years
7th dasa: Saturn Period: 70-120 years Duration: 50 yerars
By contemplating a while on these dasa-periods one can make some very interesting observations. Obviously this dasa-system is very much on-line with the traditional methods of Ayurdaya or determination of longevity.
According to Varaha Mihira the maximum longevity for human beings and elephants in Kali Yuga is 120 years and 5 days, and this dasa-system has a total duration of 120 years.
Then please observe that this dasa-system begins with the Moon,
significator of early childhood, Chandra-lagna, and the concluding
period is that of Saturn, significator of 8th house (death and
longevity).
Also please observe the exact years in which the different periods take over:
The period upto the age of 12 is covered by the Moon, Mars and Mercury. The first 12 years are generally considered to be the period of Balaristha meaning infantile mortality in Hindu Astrology.
The period from 12 upto 32 years is considered the short-life period, and this period is exactly covered by the 20 years of Venus-period in this dasa-system.
The period from 32 to 70 years is considered to be the period of medium span of life, and it is covered by Jupiter and the Sun.
Finally the period from 70 to 120 years is considered to be the period of full and long span of life, and it is covered by Saturn.
In the same way some kind of social observations can also be made from these periods.
Most people start their basic school education between 3 and 12 years in the period of Mercury.
People are then married and settled in family-life between 12 and 32 years in the period of Venus.
In the same way most people have their maximum career-expansion between age of 32 and 50 in Jupiter’s period.
They become seniors, superior or experienced workers between 50 and 70 in the period of the Sun, and get retired after 70 during Saturn Dasa.
I have made a lot of experiments with this Dasa-system, it works very well, but the things that happen in the different Naisargika Dasas greatly depend upon the planets position in the 12 houses from either Janma-lagna, Chandra-lagna or from the Dasa-planets itself. I very much like to use this Dasa-system side by side with Vimsottari Dasa.
Female person, born April 19th, 1930, Time 08:00AM,
Zone 1:00,
Copenhagen, Longitude 12E35, Latitude 55N42,
Lahiri-ayanamsa
Asc 10:26
Gem
Sun 05:38
Ari
Moon 17:41
Sgt
Mars 02:55
Psc
Merc 23:07
Ari
Jup 22:06
Tau
Ven 23:22
Ari
Sat 18:59
Sgt
Rahu 09:56
Ari
Ketu 09:56
Lib
From the age of 19 upto 29 this woman was running Moon-period according to Vimsottari-dasa. In this period she got married and gave birth to two good sons.
Why did this happen?
Well, according to my teacher the late veteran astrologer, Manik Chand Jain, it was because Moon was in the 7th house placed in Poorvashadha Nakshatra, ruled by karaka of marriage, Venus, who is also the lord of the 5th house (children), and placed in the 5th house from dasa-lord Moon with exalted Sun (sons).
This is a good example of exactly how Manik used Vimsottari Dasa. He would always use the Nakshatra-lords like this and always he would use the Dasa-lord as if it was the Ascendant.
To get a second opinion on this, we can take the dasa period of Venus in Naisargika-dasa that always runs from age of 12 upto 32 years in case of all human beings. Then we can observe if and how Venus is connected to the 5th and 7th houses from Janma-lagna, Chandra-lagna and from its own position.
From Janma-lagna, Venus is in the 11th as lord of the 5th. From Chandra-lagna, Venus is in the 5th with the 7th-lord Mercury. From its own position in Aries, Venus rules the 7th house and is with the exalted Sun who is the lord of the 5th.
This way the Naisargika Dasa actually confirms the results of the Vimsottari Dasa very nicely.