Pillar article
Mahadasha and Antardasha: The 120-Year Rule
Vimshottari antardasha length is the mahadasha lord's years multiplied by the antardasha lord's years, then divided by 120. The nine mahadashas are Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19, and Mercury 17. They sum to 120. The lord who opens a life is the lord of the Moon's nakshatra, and the first period is only the unused fraction of that nakshatra.
Where the sequence starts
The twenty-seven nakshatras are assigned to the nine lords in order, three full rounds. Ashwini is Ketu, Bharani Venus, Krittika the Sun, Rohini the Moon, Mrigashira Mars, Ardra Rahu, Punarvasu Jupiter, Pushya Saturn, Ashlesha Mercury, and Magha returns to Ketu. At birth the Moon has already covered part of its nakshatra. The remaining fraction times the lord's full years is the balance of the first mahadasha. A Moon at the end of Rohini has little Moon dasha left and soon enters Mars. A Moon at the start of Rohini can carry most of the ten-year Moon mahadasha. That balance is why two people with the same Moon sign can be in different periods.
Antardasha arithmetic
Inside a mahadasha the nine antardashas run in the same order, beginning with the mahadasha lord. Venus mahadasha starts with Venus antardasha, then Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury, and Ketu. Venus-Venus lasts 20 times 20 divided by 120, which is 3 years and 4 months. Venus-Sun lasts 20 times 6 divided by 120, which is 1 year. Saturn-Mercury lasts 19 times 17 divided by 120, a little over 2 years and 8 months. The same formula fixes every sub-period. Pratyantar dasha repeats the idea one level down. You do not need a new philosophy for each level. You need the birth nakshatra and the arithmetic.
What a period can and cannot do
A mahadasha delivers the houses its lord rules and occupies, plus the houses it aspects. Saturn aspects the 3rd, 7th, and 10th. Jupiter aspects the 5th, 7th, and 9th. Mars aspects the 4th, 7th, and 8th. A beautiful yoga sleeps until one of its grahas is mahadasha or antardasha lord. A difficult house wakes up on the same rule. Gochar can trigger a month. It does not replace the natal period. Jyothish SI computes the running lords from the Lahiri Moon after the city and time are entered. No page should name your current antardasha before that calculation.
Frequently asked questions
How long is Venus antardasha inside Saturn mahadasha?
Saturn's 19 years times Venus's 20 years, divided by 120, is 3 years and 2 months. The same product rule fixes every other antardasha.
Why is the opening mahadasha often shorter than the full span?
Only the unused part of the Moon's birth nakshatra remains. The balance is that fraction times the nakshatra lord's full years.
Do transits overrule the dasha?
Transits time triggers, especially Saturn and Jupiter from the Moon. The natal promise is delivered by the mahadasha and antardasha already running.
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D1, D9, D10, dashas, yogas, and transits, the same calculations referenced in this article.