How Astrologic computes a chart

Astrologic uses the Swiss Ephemeris with the Lahiri (Chitrapaksha) ayanamsa. Positions are sidereal, not tropical. That single choice is why a Vedic Moon sign often differs from a Western one by about one sign.

Ephemeris

Planetary longitudes come from pyswisseph. When the sepl_18.se1 and semo_18.se1 files are present, Swiss Ephemeris files are used; otherwise the engine falls back to the built-in Moshier ephemeris, which is less accurate and is not what production should run. True Node is used for Rahu; Ketu is Rahu plus 180°.

Ayanamsa

swe.set_sid_mode(swe.SIDM_LAHIRI). Lahiri is the ayanamsa of the Rashtriya Panchang. At present it is approximately 24°. We return the exact ayanamsa of the birth Julian day in the chart payload so you can compare it with another program.

Houses

Primary houses are whole-sign, counted from the sidereal Lagna. This is the classical Jyotish convention: the rising sign is house 1 in its entirety. The engine also computes sidereal Placidus cusps (tropical Placidus minus ayanamsa) and reports a chalit house number per planet. Both numbers are shown on the free kundli; the report uses whole-sign as the narrative frame.

Navamsa (D9)

Each 30° sign is divided into nine parts of 3°20′. The sequence starts from Mesha for fire signs, Makara for earth, Tula for air, and Karka for water — the standard Parāśara mapping. A planet that occupies the same sign in D1 and D9 is vargottama.

Vimshottari dasha

Period lengths are the classical 120-year set (Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19, Mercury 17). The birth mahadasha is the lord of the Moon's nakshatra; the unelapsed fraction of that nakshatra is the unelapsed fraction of that mahadasha. Year length is 365.2425 days. Other software sometimes uses 365.25; that is the usual source of small date drift.

Ashtakoot

Gun milan is scored from both Moon nakshatras and Moon signs across the eight kootas totalling 36. Directional kootas (Varna, Vashya, Tara) treat the first chart as the traditional boy seat. The score is a traditional screen, not a prediction. Moon-sign-only pages score bhakoot and graha maitri out of 12 and say so.

Time and place

Birth time is interpreted in the IANA timezone of the birth coordinates, not the browser timezone. Coordinates come from a curated Indian city table first, then a geocode cache, then Nominatim. If you pass lat/lng directly, geocoding is skipped.

How to verify

Generate a chart on the kundli tool and compare sidereal longitudes with DrikPanchang or Astrodienst using Lahiri. Agreement within a few minutes of arc is expected when both programs use Swiss Ephemeris files. House numbers will only match if the other program is also on whole-sign.

This page is the method. It is not a claim of mystical authority.