What Is a Tithi?

The Definition

A tithi (તિથિ) is the time it takes for the Moon to gain exactly 12 degrees on the Sun in longitude.

Start at the new moon, when the Moon and Sun share the same longitude — an angular separation of zero. The Moon then pulls away at roughly 12 degrees a day. Each 12-degree step it completes is one tithi. Thirty of them, totalling 360 degrees, bring the Moon all the way back to conjunction with the Sun, and that is one lunar month.

So a tithi is a measure of the Moon's progress, not a slot in a day. It happens to average about 23 hours and 37 minutes, close enough to a day that treating them as days works most of the time. But only most of the time.

The Thirty Tithis

Fifteen tithis make up the bright fortnight (sud) and fifteen the dark fortnight (vad). The first fourteen of each are counted the same way; only the fifteenth changes name.

Most Gujarati festivals are named directly from this list. Ganesh Chaturthi is Bhadarvo sud choth. Janmashtami is Shravan vad aatham. Every ekadashi is an agiyaras. Once the names are familiar, a festival date tells you where in the lunar month you are without any further lookup.

Why a Tithi Skips or Repeats

This is the part that confuses people, and the explanation is straightforward once you accept that a tithi is an angle.

The Moon's orbit is elliptical, so it does not travel at a constant speed. Near perigee it races ahead; near apogee it dawdles. The Earth's orbit varies too. The upshot is that a tithi can be as short as about 19 hours or as long as about 26 hours, against a mean of roughly 23 hours 37 minutes.

Panchangs conventionally label a calendar day with the tithi that is running at sunrise. Now consider the two edge cases:

• Kshaya (skipped) tithi. A short tithi begins after one sunrise and ends before the next. It never holds a sunrise, so no calendar day is named after it, and the calendar appears to jump from, say, satam straight to nom.
• Vruddhi (repeated) tithi. A long tithi spans two consecutive sunrises. Two calendar days are then named after the same tithi, and you see teras twice in a row.

Why Two Calendars Can Disagree by a Day

Because a tithi begins and ends at a clock time rather than at midnight, a single tithi normally covers parts of two calendar days. Deciding which day "owns" it is a matter of rule, and the rules vary.

The common default is the tithi at sunrise. But many observances have their own rule: some vrats follow the tithi present at a particular time of day, some fasts are kept on the day the tithi ends rather than the day it starts, and different sampradayas have historically settled these questions differently.

The practical consequence: two reputable panchangs can print the same festival on different dates and both be right within their own tradition. If your family has always followed a particular panchang or a particular temple's calendar, keep following it. A published app is a convenience, not an authority over your family's practice.

Reading the End Time

A good panchang gives not just the tithi but the moment it ends. A line such as Posh sud teras, ending 22:48 tells you that teras is running now and chaudas begins at 22:48 tonight. That matters when you are choosing a time for a ritual rather than simply naming the day.

The same applies to the nakshatra, which also ends at a specific time — see nakshatras and janma rashi. Between the tithi, the nakshatra and the choghadiya, you can place an event quite precisely.

Frequently Asked Questions