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Dasha System (Vimsottari)

The Vimsottari Dasha is the primary planetary period system in Jyotish. It assigns 120 years across 9 planets based on the Moon's nakshatra at birth. Each planet rules a Maha Dasha (major period) which is further subdivided into Bhukti (sub-period), Antara (sub-sub), and Pratyantara (sub-sub-sub).

Planet Periods

Planet Maha Dasha Duration
Ketu 7 years
Venus 20 years
Sun 6 years
Moon 10 years
Mars 7 years
Rahu 18 years
Jupiter 16 years
Saturn 19 years
Mercury 17 years
Total 120 years

DasaLevel Enum

Value Level
MAHA Major period (~years)
BHUKTI Sub-period (~months)
ANTARA Sub-sub-period (~weeks)
PRATYANTARA Sub-sub-sub-period (~days)

Functions

get_nakshatra_ruler

get_nakshatra_ruler(nakshatra_number: int) -> str

Returns the planet code that rules the given nakshatra (1–27). The sequence of rulers repeats the 9-planet cycle in the order: Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury.

calculate_dasa_start_planet_and_fraction

calculate_dasa_start_planet_and_fraction(
    nakshatra_number: int,
    nakshatra_pada: int
) -> tuple[str, float]

Determines the ruling planet of the first Maha Dasha after birth and the fraction of that period already elapsed at birth (based on which pada the Moon occupied within its nakshatra).

calculate_current_dasa_period

calculate_current_dasa_period(
    birth_date: date,
    current_date: date,
    birth_nakshatra: int
) -> DasaPeriod

Computes the currently running dasha period at current_date.

Returns: DasaPeriod with fields:

Field Type Description
maha_dasha str Current Maha Dasha planet code
bhukti str Current Bhukti planet code
antara str Current Antara planet code
maha_start date Maha Dasha start date
maha_end date Maha Dasha end date
bhukti_start date Bhukti start date
bhukti_end date Bhukti end date

get_dasa_sequence_from_planet

get_dasa_sequence_from_planet(
    planet: str,
    levels: int = 1
) -> list[tuple[str, float]]

Returns the ordered sequence of sub-period planets and their duration (as a fraction of the parent period) starting from the given planet.

  • levels=1 returns Bhukti sequence within a Maha Dasha.
  • levels=2 returns Antara sequence.

calculate_dasa_change_dates

calculate_dasa_change_dates(
    birth_date: date,
    birth_nakshatra: int,
    future_years: int = 10
) -> list[dict]

Projects all Maha Dasha and Bhukti transitions for the next future_years years from today. Useful for predictive timeline generation.

Returns: List of dicts with planet, level, start_date, end_date.

get_dasa_interpretation

get_dasa_interpretation(planet: str, level: DasaLevel) -> str

Returns a narrative interpretation string for a planet's Dasha at the given level (Maha / Bhukti / Antara).

Sequence Example

For a native born with Moon in Rohini (nakshatra 4, ruled by Moon):

Moon Maha Dasha → Mars Bhukti → Jupiter Antara → ...

The fraction elapsed at birth is derived from how deep into Rohini the Moon is (0–13°20′ span).

Precision & Accuracy vs Traditional Platforms

Why NDAstro gives different dates

NDAstro API uses NASA JPL DE440T via Skyfield for all Moon position calculations — the most accurate modern ephemeris. Traditional platforms (JHora, DrikPanchang, AstroSage) share two systematic deviations that cause their dasa dates to be earlier than NDAstro’s:

Deviation 1 — IAU-1940 Lahiri constants: They use Swiss Ephemeris SIDM_LAHIRI_1940 (epoch constants published in 1940) instead of modern SIDM_LAHIRI, producing an ayanamsa ~53 arcsec lower. A lower ayanamsa increases the sidereal Moon longitude by 53 arcsec.

Deviation 2 — Double Delta-T (software bug): Their SE wrapper pre-converts birth UT to TT manually, then calls swe.calc_ut() which adds Delta-T again (~69 s in 2025). This advances the Moon by a further ~43 arcsec.

Combined: ~96 arcsec extra Moon advance = ~3.9 days earlier Sun Mahadasa start.

Expected offsets from NDAstro "lahiri" mode

Dasa Lord Period Offset (traditional platforms vs NDAstro)
Sun 6 years ~3.9 days earlier
Mars / Ketu 7 years ~4.6 days earlier
Moon 10 years ~6.5 days earlier
Jupiter 16 years ~10.5 days earlier
Rahu 18 years ~11.8 days earlier
Saturn 19 years ~12.4 days earlier
Venus 20 years ~13.1 days earlier

NDAstro’s "lahiri" dates are astronomically correct. Traditional platforms’ dates are systematically shifted by the older epoch constants and the double-DeltaT bug.

true_lahiri compatibility mode

To reproduce JHora/DrikPanchang/AstroSage dasa dates for client comparison, pass ayanamsa="true_lahiri" to the dasa endpoint. This replicates both deviations (Skyfield IERS Delta-T + IAU-1940 constants) and produces dates within ~4 hours of traditional platforms. No additional dependencies are required.

DasaPeriod Model

@dataclass
class DasaPeriod:
    maha_dasha: str       # e.g. "MO"
    bhukti: str           # e.g. "MA"
    antara: str           # e.g. "JU"
    maha_start: date
    maha_end: date
    bhukti_start: date
    bhukti_end: date
    antara_start: date
    antara_end: date