Calculation Methodology
Transparency • Accuracy • Reliability
📖 Introduction
TamilCalendar.in combines modern astronomical software with traditional astrological principles to provide accurate Panchangam information. This page explains our calculation methodology in full transparency.
✓ Our Accuracy
Planetary positions: 0.001 arc-second precision | Tithi/Nakshatra: 1-2 minute accuracy | Sunrise/Sunset: 1 minute accuracy
🔭 Swiss Ephemeris
Swiss Ephemeris is a world-class astronomical library developed by Astrodienst AG of Switzerland. It calculates planetary positions based on NASA JPL (Jet Propulsion Laboratory) DE431 planetary ephemeris data.
🎯 Swiss Ephemeris Features
• Based on NASA JPL DE431 data
• 0.001 arc-second planetary position precision
• Calculations from 13000 BCE to 17000 CE
• Global standard for astrological software
• Free open source library (GPL)
We use Swiss Ephemeris through the Python pyswisseph library. For every Panchangam calculation, we obtain precise positions of the Sun and Moon from this library.
📐 Lahiri Ayanamsha
Ayanamsha is the difference between the tropical (Western) zodiac and the sidereal (Indian) zodiac. Due to Earth's axial precession, this difference increases by about 50 arc-seconds every year.
📜 Why Lahiri Ayanamsha?
• Officially adopted by the Government of India in 1955
• Recommended by Calendar Reform Committee
• Determined by N.C. Lahiri
• Most widely used Ayanamsha in India
• 2026 value: approximately 24°10'
Example: Sun tropical 30° → Sidereal 30° - 24°10' = 5°50' (Aries)
⚖️ Vakya vs Drik (Thirukanitham)
There are two major calculation methods in Tamil Panchangam tradition. We support both methods.
| Aspect | Vakya | Drik (Thirukanitham) |
|---|---|---|
| History | About 1600 years old | Modern astronomical basis |
| Calculation Basis | Ancient formulas + Bija corrections | Swiss Ephemeris + Lahiri |
| Ayanamsha | Vakya Ayanamsha (slight variation) | Lahiri Ayanamsha (Govt. approved) |
| Difference | Average 5-30 minutes | |
| Popularity | Tamil Nadu, traditional temples | All of India, Government Panchangam |
⚠️ Important Note
Both methods are valid! Follow your family tradition. TamilCalendar.in shows Drik by default, but you can switch to Vakya method.
🌟 Panchangam - Five Limbs
Panchangam (Sanskrit: पञ्चाङ्ग) means "five limbs." These five elements determine the astrological qualities of each day.
📊 Calculation Formulas
1. Tithi Calculation
Tithi is the angular difference between the Moon and the Sun. Every 12° difference equals one Tithi.
Example: Moon 100°, Sun 50° → (100° - 50°) / 12° = 4.17 → Tithi 5 (Panchami)
2. Nakshatra Calculation
Nakshatra is the lunar mansion where the Moon resides. Each 13°20' (360°/27) is one Nakshatra.
Example: Moon 45° → 45° / 13.333° = 3.37 → Nakshatra 4 (Rohini)
3. Yoga Calculation
Yoga is the sum of Sun and Moon longitudes. Each 13°20' is one Yoga.
Example: Sun 50° + Moon 100° = 150° → 150° / 13.333° = 11.25 → Yoga 12 (Vruddhi)
4. Karana Calculation
Karana is half of a Tithi. Each Tithi has two Karanas.
Fixed Karanas: Shakuni, Chatushpada, Nagavaan, Kimstughna (at month end)
🌅 Sunrise / Sunset
Accurate sunrise/sunset is essential for Rahu Kalam, Nalla Neram, and Hora calculations. We calculate these based on the user's geographical location (latitude, longitude).
📍 Location-Based Calculations
• Based on user location (GPS / city selection)
• Sunrise: Moment when Sun is 0.833° below horizon
• Atmospheric refraction accounted for
• 1 minute accuracy guaranteed
✅ Accuracy Commitment
What We Guarantee:
• Planetary positions: Swiss Ephemeris 0.001 arc-second precision
• Tithi/Nakshatra start times: 1-2 minute accuracy
• Sunrise/Sunset: 1 minute accuracy
• Rahu Kalam/Nalla Neram: Location-specific calculation
⚠️ Limitations
• Minor differences may exist between Vakya and Drik methods
• For important rituals, consult local Panchangam and astrologer
• Weather and geographical changes may affect solar times
Last Updated: 25 February 2026