What is a tithi?
Tithi is the distance between Moon and Sun, measured as an angle along the ecliptic. Each time the Moon moves 12° further from the Sun, a new tithi begins. 30 tithis make one complete lunar month, from new moon to new moon.
Panchangam means "five limbs", vaara (weekday), tithi (lunar day), nakshatra (lunar mansion), yoga, and karana. Of the five, tithi is the most widely used. It is tithi that determines when festivals fall, when to fast, and when important rituals are performed. When a Tamil calendar says "today is Ekadasi", it is a tithi being named.
The 30 tithis and the two pakshas
The lunar month is divided into two halves called pakshas. Shukla paksha (வளர்பிறை, waxing moon) runs from new moon to full moon, tithis 1 to 15. Krishna paksha (தேய்பிறை, waning moon) runs from full moon to new moon, tithis 16 to 30.
| No. | Tamil name | Transliteration | Paksha |
|---|---|---|---|
| 1 | பிரதமை | Prathamai | Shukla (waxing) |
| 2 | துவிதியை | Dvithiyai | Shukla |
| 3 | திருதியை | Thrithiyai | Shukla |
| 4 | சதுர்த்தி | Chaturthi | Shukla |
| 5 | பஞ்சமி | Panchami | Shukla |
| 6 | சஷ்டி | Sashti | Shukla |
| 7 | சப்தமி | Sapthami | Shukla |
| 8 | அஷ்டமி | Ashtami | Shukla |
| 9 | நவமி | Navami | Shukla |
| 10 | தசமி | Dashami | Shukla |
| 11 | ஏகாதசி | Ekadasi | Shukla |
| 12 | துவாதசி | Dvadashi | Shukla |
| 13 | திரயோதசி | Thrayodashi | Shukla |
| 14 | சதுர்த்தசி | Chaturdashi | Shukla |
| 15 | பௌர்ணமி | Pournami | Full moon |
| 16 | பிரதமை | Prathamai | Krishna (waning) |
| 17 | துவிதியை | Dvithiyai | Krishna |
| 18 | திருதியை | Thrithiyai | Krishna |
| 19 | சதுர்த்தி | Chaturthi | Krishna |
| 20 | பஞ்சமி | Panchami | Krishna |
| 21 | சஷ்டி | Sashti | Krishna |
| 22 | சப்தமி | Sapthami | Krishna |
| 23 | அஷ்டமி | Ashtami | Krishna |
| 24 | நவமி | Navami | Krishna |
| 25 | தசமி | Dashami | Krishna |
| 26 | ஏகாதசி | Ekadasi | Krishna |
| 27 | துவாதசி | Dvadashi | Krishna |
| 28 | திரயோதசி | Thrayodashi | Krishna |
| 29 | சதுர்த்தசி | Chaturdashi | Krishna |
| 30 | அமாவாசை | Amavasai | New moon |
Why tithi does not follow the clock
A tithi is not 24 hours. It is defined as the time the Moon takes to move 12° away from the Sun, and because the Moon's speed varies (it moves faster when closer to Earth), a tithi can last from about 19 to 26 hours. Some tithis start and end within the same calendar day; others stretch across two.
This is why you see "tithi ends at 14:32" in panchangam, it marks the instant the next tithi begins. The tithi prevailing at sunrise names the entire day.
How tithi is calculated
The calculation is surprisingly simple in principle: find the ecliptic longitudes of Moon and Sun, take the difference, and divide by 12°.
Worked example
Worked example, Chennai, 15 January 2026
- Sun's ecliptic longitude
- 294.38°
- Moon's ecliptic longitude
- 42.15°
- Elongation (Moon − Sun)
- (42.15 − 294.38 + 360) mod 360 = 107.77°
- Tithi number
- ⌊107.77 / 12⌋ + 1 = 9 + 1 = 10 → Dashami
The 10th tithi, Dashami, in shukla paksha (waxing moon).
Elongation is computed mod 360°. Tithis 1–15 are shukla, 16–30 are krishna.
Note that the calculation uses the difference between two longitudes. This makes tithi independent of ayanamsa, the sidereal correction that separates Indian and Western astronomy. Whether you use the tropical or sidereal coordinate system, the elongation gives the same value.
Our calculator uses Meeus's 60-term series for the Moon's position and the NOAA algorithm for the Sun. The end time of a tithi is computed with a secant method that finds the exact instant the elongation crosses the next 12° boundary.
The most important tithis in practice
All 30 tithis carry significance in Hindu tradition, but some are far more widely observed than others:
- Ekadasi (11th tithi), fasting day for Vishnu devotees. Falls twice a month, once in each paksha. The most widely observed tithi in practice.
- Chaturthi (4th tithi), Ganesha's day. Vinayaka Chaturthi (shukla, Bhadrapada) is one of the year's biggest festivals.
- Pournami (15th tithi), full moon. Temple festivals, ancestral rites, and the day many major festivals fall: Thai Pongal, Guru Purnima.
- Amavasai (30th tithi), new moon. Ancestral rites (pitru tarpanam). The day the Sun and Moon share the same ecliptic longitude.
- Pradosham, falls on Thrayodashi (13th tithi) in both pakshas. Shiva worship at sunset.
- Sashti (6th tithi), Murugan's day. Skanda Sashti (krishna, Aippasi) is a major festival in Tamil Nadu.
- Navami (9th tithi), Sri Rama Navami (shukla, Chaitra) and Maha Navami during Navaratri.
Tithi and location, does it change?
The tithi number itself does not change from city to city, the elongation is the same everywhere on Earth at any given instant. But because sunrise happens at different times, the tithi prevailing at sunrise can differ. A city with an early sunrise may still have yesterday's tithi, while a city with a late sunrise has already shifted to the next one.
In practice, you will rarely notice this within the same time zone, but it can happen for diaspora users in North America or Europe compared with Chennai. Our calculator handles this correctly by computing tithi at the local sunrise in the selected city.
Tithi in the Tamil calendar
The Tamil calendar is a solar calendar, months follow the Sun's position in the zodiac. But tithi still plays the leading role in religious decisions. When your grandmother says "today is Ekadasi, we fast", it is the tithi system speaking, not the solar month.
North Indian calendars (like Vikram Samvat) are lunar and use tithi as the actual date system, "Chaitra Shukla Panchami" is a full date. The Tamil tradition uses the solar reckoning for dates but tithis for all religious observances.