The Science Behind
Hindu Calendar & Location-Specific Calculations
Understand why Hindu festivals and tithis change based on your location. Learn the astronomy, mathematics, and spiritual significance behind accurate calendar calculations.
What is a Tithi?
A tithi is a lunar day in the Hindu calendar system. Unlike the Gregorian calendar where a day is 24 hours from midnight to midnight, a tithi is based on the angular distance between the Sun and Moon as observed from Earth.
Mathematical Definition
A tithi is the time it takes for the longitudinal angle between the Sun and Moon to increase by 12 degrees. There are 30 tithis in a lunar month:
- •15 tithis in the waxing phase (Shukla Paksha) - from new moon to full moon
- •15 tithis in the waning phase (Krishna Paksha) - from full moon to new moon
Visual Explanation
As the Moon orbits Earth, the angle between Sun and Moon increases. Each 12° increment = 1 tithi.
Important Tithis
Day of fasting dedicated to Lord Vishnu. Breaking fast (parana) timing is critical and location-specific.
Full moon day. Important for worship and many festivals like Holi, Guru Purnima.
New moon day. Significant for ancestor worship and festivals like Diwali.
Day before Purnima/Amavasya. Important for certain rituals and festivals.
Why Location Matters for Tithi Calculations
This is the most critical concept to understand: Tithis are calculated based on your exact geographic location (latitude and longitude), not just your timezone.
Common Misconception
Many people think you can simply convert Indian time to your local timezone. This is incorrect. Tithi calculations require recalculating the entire Panchang using your exact coordinates.
The Science Behind It
1. Geocentric Calculations
Tithis are calculated using the geocentric positions of the Sun and Moon - their positions as seen from Earth. While these positions are the same globally at any moment, the local observation time differs.
2. Sunrise-Dependent Day
In Hindu calendar, a day begins at sunrise, not midnight. This means the same tithi can span two different Gregorian calendar days in different locations.
3. Angular Distance
The 12-degree angular increment that defines a tithi is measured from your specific location on Earth. Your latitude and longitude affect when this angle is observed.
Example: Same Tithi, Different Dates
Tithi begins: 7:15 AM IST (after sunrise)
Festival date: November 13
Same tithi begins: 3:30 AM EST (before sunrise)
Festival date: November 12
Both dates are scientifically correct for their respective locations. The tithi is the same astronomical event, but the local day (determined by sunrise) differs.
How Festivals Are Determined
Most Hindu festivals are determined by specific tithis (lunar days). Since tithis are location-specific, festival dates also vary by location.
Festival-Tithi Relationship
Celebrated on Amavasya tithi (new moon) of Kartik month. Since Amavasya occurs on different dates in different locations, Diwali dates vary.
Fasting day on the 11th tithi of both waxing and waning phases. Each Ekadashi has a specific name and occurs on different dates based on location.
Celebrated on Purnima tithi (full moon) of Phalgun month. The full moon date varies by location.
Begins on Pratipada tithi (1st tithi) of specific months. The start date varies based on when Pratipada begins in your location.
Why Festivals Change by Location
Tithi-Based Festivals
Since festivals are tied to tithis, and tithis are location-specific, festival dates automatically vary by location.
Sunrise Determines the Day
The Hindu day begins at sunrise. If a tithi begins before sunrise in one location but after sunrise in another, the festival falls on different dates.
Both Dates Are Correct
Celebrating Diwali on Nov 12 in New York and Nov 13 in Mumbai are both scientifically and spiritually correct for their respective locations.
Solar-Based Festivals
Some festivals like Makar Sankranti are based on solar movements (Sun entering Capricorn). These have less variation but still require location-specific calculations for precise timing.
Why Sunrise Matters: The Hindu Day
Unlike the Gregorian calendar where a day runs from midnight to midnight, the Hindu calendar day begins at sunrise and ends at the next sunrise. This fundamental difference is why location-specific calculations are essential.
Gregorian Calendar
Hindu Calendar
Example: Ekadashi Parana Timing
Ekadashi fasting must be broken (parana) after sunrise on Dwadashi (12th tithi). This timing is critical:
Using Indian sunrise time (6:58 AM IST) in New York means breaking fast at wrong time, negating spiritual benefits.
Using local sunrise time (6:42 AM EST) in New York ensures breaking fast at the correct moment for maximum spiritual benefit.
How OM Calendar Calculates Dates
OM Calendar uses NASA-grade ephemeris data (the same data used by Drik Panchang) to calculate celestial positions with high precision.
Step 1: Location
We use your exact latitude and longitude (not just timezone). This can be from GPS or manually set city coordinates.
Step 2: Celestial Positions
Using NASA JPL ephemeris data, we calculate the exact positions of Sun and Moon relative to your location at any given moment.
Step 3: Tithi Calculation
We calculate the angular distance between Sun and Moon. Each 12° increment represents one tithi. We determine when each tithi begins and ends relative to your local sunrise.
Step 4: Festival Mapping
We map specific tithis to festivals (e.g., Amavasya = Diwali, Purnima = Holi). The festival date is determined by which Gregorian calendar day the tithi falls on based on your local sunrise.
Why This Matters
Scientific Accuracy: Uses the same calculation methods as trusted Panchang sources like Drik Panchang.
Location Precision: Accounts for your exact coordinates, not just timezone approximation.
Spiritual Correctness: Ensures you observe festivals and fasts at the correct time for maximum spiritual benefit.
Real-World Examples
Example 1: Diwali
Diwali is celebrated on Amavasya tithi of Kartik month. The Amavasya tithi begins at different times in different locations.
Amavasya begins: 7:15 AM
Diwali: Nov 13
Same Amavasya begins: 3:30 AM
Diwali: Nov 12
Same Amavasya begins: 2:45 AM
Diwali: Nov 12
Key Insight: All three dates are correct for their locations. The tithi is the same astronomical event, but local sunrise determines which Gregorian date it falls on.
Example 2: Ekadashi Parana
Ekadashi fast must be broken after sunrise on Dwadashi (12th tithi). The parana timing is critical and location-specific.
Result: Breaks fast at wrong time, negating spiritual benefits
Result: Breaks fast at correct time for maximum spiritual benefit
References & Sources
Academic & Scientific Sources
Comprehensive explanation of the lunisolar calendar system, tithis, and festival calculations.
View SourceHindu Heritage Services explains why festival dates vary by location and how tithi calculations work globally.
View SourceCalculation Methodology
OM Calendar uses NASA's Jet Propulsion Laboratory ephemeris data for calculating celestial positions. This is the same data used by Drik Panchang and other trusted Panchang sources.
View SourceOM Calendar calculations are verified against Drik Panchang, a trusted source for Panchang calculations used by millions of Hindus worldwide.
View SourceKey Concepts Explained
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OM Calendar automatically calculates accurate tithi dates and festivals for your exact location. No manual calculations needed.