What a dasha is

A dasha is a period of time assigned to a graha. The Vimshottari system, which is the one nearly every Tamil astrologer means by the word, divides 120 years among nine grahas in fixed shares and runs them in a fixed order.

The shares are Ketu 7 years, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19 and Mercury 17. They add to 120, and they always run in that order, wrapping round at the end.

What varies from person to person is only where in that cycle you are born. That is decided by the Moon: which nakshatra it stands in, and how far through that nakshatra it has travelled.

Each of the 27 nakshatras has a lord, taken from the nine in the order above and repeating every ninth star. So the first, tenth and nineteenth stars belong to Ketu, the second, eleventh and twentieth to Venus, and so on.

Take the Moon’s sidereal longitude at the moment of birth. Divide by 13°20′ to get the nakshatra, and keep the remainder — the fraction of that star already crossed. That fraction, multiplied by the lord’s own share of years, is how much of the first mahadasha had already run before the birth.

A Moon a quarter of the way into Rohini, whose lord is the Moon with ten years, is born with two and a half years of the Moon mahadasha already spent and seven and a half left. That remainder is the balance every printed horoscope starts with.

The rest is laid out end to end from there: the balance of the first lord, then the next lord’s full share, and so on until 120 years are used up. The app then subdivides each period the same way — a mahadasha’s children in the same proportions, opening with the period’s own lord, and again below that, five levels deep to the prana dashas. There are nine to the fifth power of those, 59,049, so they are built only along the paths a reader actually opens.

A dasha "year" has to be turned into days before a date can be printed, and the classical texts do not fix the conversion. This app uses 365.25 days by default, which is what most Indian dasha software and most printed dasha tables use.

Three other lengths can be chosen: the mean civil year of 365.2425 days, the sidereal year of 365.256363 days, and the savana year of 360 days that some Parashari teachers use. Jagannatha Hora makes the same choice a user setting, and its author argues for the sidereal one.

Over a lifetime the difference is not small. A 19-year Saturn mahadasha is about 100 days shorter on the savana year than on the Julian one. If your dates and another program’s dates differ by a few months, this is the first thing to check, and it is not an error in either.

The Moon moves about half a degree an hour. A nakshatra is 13°20′ wide, so the Moon crosses a whole star in a little over a day, and one minute of clock is roughly one part in fifteen hundred of a star.

That fraction is then multiplied by the lord’s years, which is where a small number becomes a visible one. Running the app’s own chart and dasha code over about twelve hundred birth instants a minute apart gives the measured answer: one minute of birth time moves every boundary in the table by between about 1.4 days and about 5.8 days.

Which end you land on depends on which graha owns the birth star. A Sun star, with six years, gives about 1.4 to 1.8 days per minute. A Ketu or Mars star, with seven, gives about 1.6 to 2. A Venus star, with twenty, gives about 4.5 to 5.8. Saturn and Rahu are close behind Venus.

So four minutes of uncertainty — the amount that moves the ascendant a degree — is somewhere between five days and three weeks on every dasha date in the chart. A quarter of an hour is weeks to a couple of months. And because the whole sequence is laid out from that one starting offset, the shift is rigid: every mahadasha, every antardasha and every level below moves together, by the same amount.

This is why a birth time written on a certificate is worth more than a remembered one, and why the app carries a rectification tool that narrows an uncertain time from dated life events rather than guessing at it.

Three honest reasons, and none of them is an error.

They may be using a different dasha system. Vimshottari is the default, but it is not the only one. Ashtottari divides 108 years among eight lords, with the stars grouped differently and Abhijit counted in. Yogini divides 36 years among eight, so the whole cycle runs out inside a lifetime. Kalachakra is a sign dasha opened by the Moon’s pada, and Parashara calls it the most respected of them. Narayana, Sthira and Mandook are Jaimini sign dashas: they run through the twelve signs rather than the grahas, and each is opened from the lagna, so each needs a birth time absolutely. All of them are in this app, and all of them will tell you a different graha is running today.

They may be using a different dasha year, as above.

They may be working from a different ayanamsa, which moves the Moon’s sidereal longitude and therefore the fraction the clock starts from — and, near a star boundary, the starting lord itself.

None of those is a disagreement about the sky. They are disagreements about convention, and each of them is a setting you can match.

The arithmetic is exact and has been checked. The app’s Vimshottari dates were compared against an independent reimplementation using exact rational arithmetic over 2,000 charts; the mahadasha, antardasha and pratyantara dates agree to about 1.4 billionths of a day, which is a tenth of a millisecond. Ashtottari, Yogini, Kalachakra and Chara were checked the same way.

What that establishes is narrow and complete: given a Moon position, a starting convention and a dasha year, the periods are laid out correctly. That is a claim about arithmetic.

That 120 years is the span of a life, and that Saturn’s nineteen of them mean what the texts say they mean, is not something arithmetic reaches. This app will tell you which period is running and what the tradition says about it. It will not tell you what is going to happen in it.

Every level of every dasha, and what changes on a date.

Dasha →