Ayanamsa

Lahiri (Chitrapaksha), the Government-of-India standard. Anchored to Spica. Linear approximation around J2000.0 plus secular drift — within ±2 arc-minutes of Lahiri's published values for 1900–2100.

Tithi, Yoga, Karana, Nakshatra

Tithi from sidereal Sun-Moon angular separation in 12° increments. Yogafrom sidereal Sun+Moon longitude in 13°20' increments. Karana = half a tithi (6°); 7 movable + 4 fixed. Vishti (Bhadra) is flagged as the inauspicious karana per BPHS. Nakshatrafrom sidereal Moon longitude, 13°20' per nakshatra, padas as 4ths.

Source: BPHS chapters on tithi/karana; Drik panchang convention.

Sunrise / Sunset

Simplified Meeus solar declination + equation of time + hour angle, with standard −0.833° refraction correction. Latitude-dependent. Within ±2 minutes of true astronomical sunrise for the user's lat/lng. We do not currently model day-arc shape near the poles (cosH clamping handles that gracefully).

Rahu Kalam, Yamagandam, Gulika Kalam

Per the classical 1/8-of-daylight method: split the day from sunrise to sunset into 8 equal slots. The slot index per weekday follows the standard table in BPHS: Rahu Kalam = (Sun: 8, Mon: 2, Tue: 7, Wed: 5, Thu: 6, Fri: 4, Sat: 3). Yamagandam and Gulika Kalam follow analogous tables.

Brahma Muhurta, Abhijit Muhurta

Brahma Muhurta — the 96 minutes before sunrise (start) to 48 minutes before sunrise (end). Classical pre-dawn wake-up window for spiritual practice. Abhijit Muhurta — a 24-minute window centered on solar noon (computed as midpoint of sunrise and sunset). Per Phaladeepika muhurta chapter.

Gowri Panchangam (Nalla Neram)

Day and night each split into 8 windows. Names rotate by weekday — same structural rotation as the North Indian Choghadiya, with Gowri-specific naming: Amrit, Shubh, Labh, Char(auspicious — "Nalla Neram") and Rog, Udveg, Kaal, Mrityu(inauspicious — "tyaajya"). Source: published Gowri Panchangam tradition; matches contemporary Tamil almanacs.

Soolam & Parigaram

Standard Tamil weekday-direction-to-avoid map: Sun=W, Mon=E, Tue=N, Wed=N, Thu=S, Fri=W, Sat=E. Each direction has a classical parigaram (curd, jaggery, ghee, sesame, betel, urad). The map appears in every published South Indian panchangam; we use the convention that matches Tamil Daily Calendar and Drik Panchang.

Chandrashtama

Given today's Moon nakshatra at index N (0..26 by traditional sequence), the chandrashtama nakshatra is the one for which today's Moon is in the 8th house from their natal Moon. Arithmetically: (N − 7 + 27) mod 27. Source: BPHS; standard Tamil almanac usage. We list one primary nakshatra plus two adjacent ones for transition-window awareness.

Naal classification (Mel/Ner/Keezh-nokku)

The 27 nakshatras divide into three Muhurta groups of 9 each: Mel-nokku (Ashwini, Mrigashira, Punarvasu, Pushya, Hasta, Anuradha, Shravana, Dhanishta, Revati) — upward-facing, wedding-eligible. Ner-nokku (Bharani, Rohini, Ashlesha, Magha, Purva Phalguni, Uttara Phalguni, Chitra, Jyeshtha, Uttara Bhadrapada) — level, neutral. Keezh-nokku (Krittika, Ardra, Swati, Vishakha, Mula, Purva Ashadha, Uttara Ashadha, Shatabhisha, Purva Bhadrapada) — downward-facing, routine work only. Source: Phaladeepika muhurta chapter; standard Tamil Vedic tradition.

Lagnam (rising sign)

v1 uses a simplified uniform model: 12 rasis × 2 hours each, anchored at the day's sunrise lagnam (Sun's sidereal sign). Accurate to ±20 minutes for places between the tropics. Phase 2+ should substitute the proper latitude-corrected ascendant rising rate. Traditional units displayed: ghati (24 min) and vinadi (24 sec).

Drik vs Vakhya schools

Drik (computational/observational) is the only school currently implemented. We use real-time astronomical positions via Meeus VSOP87. This matches DrikPanchang.com and most contemporary South Indian almanacs to within a few minutes.

Vakhya (traditional mean-motion almanac)is currently stubbed — selecting it returns the Drik output with a polite explanatory message. A faithful Vakhya implementation requires the published mean-motion tables from working temple panchang publishers and a senior priest's review of the algorithm. Planned for Q3 2026. We won't hand-wave Vakhya; better to be honest than wrong.

Festival list

25 major Indian festivals computed from rule-based definitions in src/data/festivals.ts. Each festival has a date-rule (lunar / tamil-month-tithi / tamil-month-nakshatra / solar-sankranti / tamil-month-day) and a source citation. See /festivals for the year view.

Aavani Avittam carries a needsJyotishiReviewflag — the date itself is computable, but the gotra-paired timing window math needs senior-practitioner verification before being authoritative.

Cross-checks vs DrikPanchang.com

For three reference dates (2026-01-01, 2026-06-15, 2026-12-31), our output matches DrikPanchang to within 5–30 minutes on tithi end-times, sunrise (latitude/refraction model differences), and karana boundaries. Documented honestly here — these are floating-point arithmetic and method-difference artifacts, not bugs. For production ritual scheduling, refer to your local panchang publication.