A tithi is a lunar day: the time the Moon takes to gain 12° on the Sun. Thirty tithis make one lunar month, fifteen in the waxing fortnight and fifteen in the waning. Because the Moon's speed varies, a tithi is not a fixed length — it runs from roughly 19 to 26 hours, which is why it does not line up with the civil day.
How a tithi is arrived at
The computation is the angular distance between the Moon and the Sun, divided into thirty parts of 12° each. When that distance is 0° the tithi is Amavasya; at 180° it is Pournami. A panchangam lists the tithi running at sunrise and the clock time at which it ends, because a tithi almost never begins or ends at a convenient hour.
The fortnights are named for the direction of the change: Shukla Paksha while the Moon waxes towards Pournami, Krishna Paksha while it wanes towards Amavasya. The numbered tithis — Padyami through Chaturdashi — repeat in both, which is why a date is always given with its paksha.
- Nanda: Padyami, Shashti, Ekadashi.
- Bhadra: Vidiya, Saptami, Dwadashi.
- Jaya: Tadiya, Ashtami, Trayodashi.
- Rikta: Chaviti, Navami, Chaturdashi — the group generally set aside for auspicious beginnings.
- Purna: Panchami, Dashami, Pournami or Amavasya.
What it decides
Almost every vratam and festival in the Hindu calendar is fixed by tithi rather than by a Gregorian date, which is why the dates move each year. Ekadashi fasts, Sankatahara Chaturthi, Pradosham, Amavasya tarpanam and the death-anniversary shraddham are all tithi observances.
The rule usually applied is that the tithi current at sunrise names the day. Several observances are exceptions and take the tithi current at the hour the ritual is performed instead — Janmashtami at midnight and Shivaratri at nishita kaal are the common ones. This is the source of most disagreements between published calendars about which day a festival falls on.
Where the general rule stops
A tithi can be skipped or repeated across two civil days, and the convention for naming the day differs between observances. Take the tithi and its end time for your date from a current panchangam computed for your place rather than assuming one tithi per day.
Every timing on this page is computed for Hyderabad and Indian Standard Time. For a consequential date, confirm the window with the Purohit conducting the ceremony rather than acting on a published table.
Written and reviewed by the AnugrahaSidhi editorial team. Last substantive review: . How we source and correct this material is set out in our editorial policy.