Tithi: the lunar day, and why it changes in the middle of the day

A tithi is a lunar day. It is defined by geometry, not by the clock: each time the Moon gains another 12° on the Sun, one tithi ends and the next begins. At new moon (Amavasya) the separation is zero; at full moon (Pournami, Purnima) it is 180°.

Thirty tithis, two fortnights

From new moon to full moon the Moon waxes through the fifteen tithis of Shukla Paksha (valar pirai); from full moon back to new moon it wanes through the fifteen of Krishna Paksha (thei pirai). The names repeat in both halves — Prathama, Dwitiya, Tritiya, Chaturthi, Panchami, Shashti, Saptami, Ashtami, Navami, Dasami, Ekadasi, Dwadasi, Trayodasi, Chaturdasi — ending in Pournami or Amavasya.

Why tithis start and end at odd times

The Moon’s speed varies as it moves round its elliptical orbit, so a tithi lasts anywhere from about 19 to about 26 hours and can change at any moment of the day or night. By convention the tithi present at sunrise is the tithi “of the day”. Occasionally a short tithi begins after one sunrise and ends before the next, so it is never the tithi of any day; sometimes a long one spans two sunrises. Different traditions resolve festival dates in these cases differently, which is why two calendars can disagree by a day.

The end times shown on this site are the true astronomical moments, which are the same instant everywhere in the world; only the local clock time differs.