A 12-degree lunar interval
A tithi begins whenever the Moon–Sun ecliptic longitude difference crosses another multiple of 12 degrees. Thirty such intervals make a complete lunar cycle. The waxing half is Shukla Paksha; the waning half is Krishna Paksha.
The first fourteen names repeat in each fortnight. The fifteenth waxing tithi is Purnima, and the thirtieth tithi is Amavasya. They end at full moon and new moon respectively.
| Number within fortnight | Name | Example of a full label |
|---|---|---|
| 1 | Pratipada | Chaitra Shukla Pratipada |
| 8 | Ashtami | Bhadrapada Krishna Ashtami |
| 11 | Ekadashi | Kartika Shukla Ekadashi |
| 15 | Purnima / Amavasya | Phalguna Shukla Purnima / Kartika Krishna Amavasya |
Why two dates can share a tithi
The Moon’s apparent speed varies, so a tithi is not exactly 24 hours long. A longer tithi can be present at two consecutive sunrises. A shorter one can begin after one sunrise and end before the next. In that case it has no sunrise date even though the interval still occurred.
This explains why a nine-night festival schedule does not always produce nine consecutive civil-date rows with nine different tithi names. In the 2026 Navratri reference, Saptami appears on 17 and 18 October. Keeping both date and tithi avoids silently renumbering the schedule.
Read the start and end, not just the day name
Select a date in the Hindu calendar. Its panel shows the tithi at the selected city’s sunrise and the calculated interval in IST. If the interval ends after midnight, the panel prints the next civil date. This is clearer than treating “25:00” as an ordinary clock reading: that notation means 1 am on the following civil day.
Near a sunrise boundary, small differences in the horizon or astronomical method can affect which tithi is recorded at sunrise. The tool flags transitions within five minutes of that boundary and retains the interval for comparison.
From tithi to festival
Festival rules may use a tithi at sunrise, evening, moonrise or another part of the day. Diwali’s evening Amavasya association is a useful example: 8 November 2026 can begin with Chaturdashi at sunrise while still being the published Lakshmi Puja date. A day’s first tithi label is only one part of the rule.
IMD Panchang explanation · Astronomical elongation reference