Tithi: the lunar day
A tithi is the time the Moon takes to gain 12 degrees on the Sun. There are 30 in a lunar month, and each has a presiding deity and its own length.
What a tithi is
A tithi = lunar day. It is the time the Moon takes to move 12 degrees ahead of the Sun, measured along the sky. At New Moon the two are together. Each time the gap grows by another 12 degrees, a new tithi begins. After 360 degrees, which is one full lunar cycle, 30 tithis have passed.
The first 15 tithis make the Shukla paksha (bright half, the Moon waxing). The next 15 make the Krishna paksha (dark half, the Moon waning). Each half has the same 15 names. The 15th tithi is Purnima (full Moon) at the end of the bright half and Amavasya (new Moon) at the end of the dark half.
Why tithis have different lengths
If the Moon moved at an even speed, every tithi would last the same time. It does not. The Moon's orbit is slightly oval, so it moves faster when nearer the Earth and slower when farther. The Sun's apparent speed varies a little as well.
The average tithi is about 23 hours 37 minutes. Actual tithis range from a little under 20 hours to nearly 27 hours. Because a tithi is shorter than a day on average, tithi numbers advance faster than the weekdays.
Tithi begins and ends at any hour
A tithi does not start at sunrise or midnight. It begins the moment the Sun and Moon reach the right angle, and the exact time differs by city only in the clock reading. This site always shows the start and end time of each tithi in your local time.
The day's tithi, used for festivals and fasts, is normally the one running at sunrise. This is the udaya tithi rule.
Kshaya and vriddhi tithis
Since tithis and days do not line up neatly, two things happen from time to time.
- Kshaya tithi = a "lost" tithi. A tithi begins after one sunrise and ends before the next, so it is never present at a sunrise. The Panchang then skips its number for that date.
- Vriddhi tithi (also called adhika tithi) = a "repeated" tithi. One tithi is running at two sunrises in a row, so the same tithi number is counted on two consecutive days.
These are normal. They do not signal anything wrong. They are why a festival can appear to move by a day.
The thirty tithis and their deities
Each tithi has a traditional presiding deity. The Shukla and Krishna paksha share the deities for the first fourteen, and the 15th differs. Sources list the deities with small variations, and some old texts give a different order, so treat this as a widely used list.
| No. | Tithi | Presiding deity |
|---|---|---|
| 1 | Pratipada | Agni |
| 2 | Dwitiya | Brahma |
| 3 | Tritiya | Gauri (Parvati) |
| 4 | Chaturthi | Ganesha |
| 5 | Panchami | Nagas (serpent deities) |
| 6 | Shashthi | Kartikeya (Skanda) |
| 7 | Saptami | Surya |
| 8 | Ashtami | Rudra (Shiva) |
| 9 | Navami | Durga |
| 10 | Dashami | Yama (Dharmaraja) |
| 11 | Ekadashi | Vishnu |
| 12 | Dwadashi | Vishnu |
| 13 | Trayodashi | Kamadeva |
| 14 | Chaturdashi | Shiva |
| 15 | Purnima (Shukla) | Chandra (the Moon) |
| 15 | Amavasya (Krishna) | The Pitris (ancestors) |
So the thirty tithis are Shukla Pratipada to Shukla Purnima, then Krishna Pratipada to Krishna Amavasya. Some lists give Yama for Chaturthi alongside Ganesha, or Vishvedevas for Ekadashi. Follow your family's tradition.
How tithis are used
Many observances are named for a tithi: Ekadashi for fasting, Chaturthi for Ganesha, Ashtami and Navami for the goddess, Amavasya for ancestors and Purnima for the full Moon. Festivals combine a tithi with a Hindu month, for example Chaitra Shukla Navami.
Birth tithi is also used in some families to mark a person's Hindu birthday, and the death tithi is used for the annual shraddha. Ask your priest which rule your family follows when a tithi spans two days.