What is Tithi?
A tithi is a lunar day: the interval during which the Moon moves 12° further ahead of the Sun. It is the first and most important limb of the panchang, and the anchor for almost every Hindu festival and vrat.
The 30 tithis
One synodic (lunar) month contains 30 tithis. The first 15, while the Moon waxes, form the Shukla Paksha (Pratipada, Dwitiya … up to Purnima, the full moon). The next 15, as the Moon wanes, form the Krishna Paksha, ending in Amavasya (new moon).
Why a tithi isn’t 24 hours
Because the Moon’s speed varies along its elliptical orbit, a tithi is not fixed — it ranges from about 19 to 26 hours. So a tithi can occasionally skip a sunrise (kshaya) or span two (vriddhi). The tithi “in force” on a day is usually the one running at sunrise.
Tithi and festivals
Festivals are defined by tithi, not by the Gregorian date — Janmashtami is Shravana Krishna Ashtami, Diwali is Ashwin Krishna Amavasya. This is why their English dates move every year.
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FAQ
How many tithis are there?
30 per lunar month — 15 in the waxing Shukla Paksha and 15 in the waning Krishna Paksha.
Why does a tithi’s length vary?
Because the Moon travels at different speeds along its elliptical orbit, the time to gain 12° on the Sun varies from roughly 19 to 26 hours.