What is a tithi

Both the Sun and the Moon travel through the zodiac, but the Moon is much faster. After the new moon it pulls ahead of the Sun. Each time the gap grows by 12 degrees, one tithi is complete. After 30 such steps the Moon has caught up with the Sun again, which is the next new moon.

The tithi running at sunrise gives the day its name. If Chaturthi is running at sunrise, the day is called Chaturthi, even if the tithi ends soon after. A tithi is not 24 hours long. The Moon's speed changes through its orbit, so a tithi runs from roughly 19 to 26 hours. This is why the panchang gives the exact end time of each tithi for your city.

The 30 tithis fall into two halves of 15, called pakshas. Shukla paksha, the bright half, runs from the new moon to the full moon while the Moon grows. Krishna paksha, the dark half, runs from the full moon to the next new moon while it fades. Each half is counted from Pratipada, the first tithi.

The names follow the same order in both halves: Pratipada, Dwitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dwadashi, Trayodashi and Chaturdashi. The fifteenth is different. In Shukla paksha it is Purnima, the full moon. In Krishna paksha it is Amavasya, the new moon. So Ekadashi of Shukla paksha and Ekadashi of Krishna paksha are two different days.

A day runs from sunrise to sunrise, but a tithi is not tied to sunrise. A short tithi can begin after one sunrise and end before the next. It then never touches a sunrise and names no day. This is called kshaya, a skipped tithi. The count of days jumps over it, for example from Tritiya straight to Panchami.

A long tithi does the opposite. If it is still running at two sunrises in a row, it names two days. This is called vriddhi, or adhik tithi, a repeated tithi. Both are normal. Tithis are not 24 hours long and the day is counted from sunrise, so now and then a tithi is dropped and now and then one is doubled.

Most vrats and festivals are tied to a tithi, not to a fixed date on the civil calendar. This is why Diwali and Janmashtami move from year to year. The tithi alone is often not enough. Each observance also names the time of day when its tithi must prevail, such as sunrise, midday, the evening or midnight.

Ekadashi shows how these rules work. If Ekadashi spans two sunrises, the Smarta fast is kept on the second day. Other vrats have rules of their own, and local custom can differ by a day from one region to another. Our panchang shows each vrat or festival on the day it is kept, worked out for your city.