What Is a Tithi? The Lunar Day Explained

Almost every date in Hindu observance is given as a tithi rather than a calendar date. Ekadashi, Chaturthi, Amavasya, Purnima — these are tithis. Understanding what one actually is explains most of what people find confusing about the Hindu calendar, including why two households can observe the same festival on different days.

In short: a tithi is a lunar day. But it is measured by the Moon’s position, not by the clock, so it does not begin at midnight or last twenty-four hours. Everything else on this page follows from that.

The definition, which is astronomical rather than traditional

A tithi is the time the Moon takes to gain twelve degrees of angular distance on the Sun.

That is the whole definition. At the new moon the Sun and Moon share the same celestial longitude, and the separation is zero. As the Moon moves ahead, that gap opens. Every twelve degrees of gap is one tithi. Since a full circuit is 360 degrees, a lunar month contains exactly thirty tithis.

It helps to picture what you see in the sky. Both bodies move eastward against the stars, but the Moon moves much faster, so it steadily pulls ahead of the Sun. A thin crescent after new moon means the gap is still small. At 90° you see a half-lit Moon; at 180° the Moon stands opposite the Sun and is full. The tithi is simply a way of counting that gap in fixed steps, which is why the phase of the Moon and the tithi always go together.

Why a tithi is not a day

This is the part that matters, and it follows from the definition rather than from custom.

The Moon does not travel at a constant speed. Its orbit is an ellipse, so it moves faster when nearer the Earth and slower when further away. The gap between Sun and Moon therefore widens at an uneven rate, and a tithi — a fixed twelve degrees of that widening — takes a variable amount of time. In practice a tithi runs roughly nineteen to twenty-six hours. On average it is a little shorter than a solar day, because thirty tithis fit into one lunar month of about twenty-nine and a half days.

Three consequences follow, and together they account for nearly every apparent inconsistency in festival dating:

• A tithi can begin and end at any clock time. It has no reason to align with sunrise or midnight. A panchang that says “Ekadashi until 10:40” means the Moon completes that twelve-degree step at 10:40, and the next tithi begins.
• A tithi can be skipped, or repeated. If a tithi begins and ends entirely between two sunrises it is a kshaya tithi and that number is missing from the calendar. If a slow tithi spans two sunrises it is an adhika tithi and the number appears twice.
• Which day a festival falls on depends on a rule, not just on the tithi. Different traditions specify a different moment for the test — the tithi current at sunrise, or at moonrise, or at midday, or at the particular watch of the night the observance belongs to. Two panchangs applying different rules to identical astronomy will print different dates, and both are correct within their own convention.

How the sunrise rule works in practice

The most common everyday rule is that the tithi current at sunrise names the whole civil day, from that sunrise to the next. Suppose Chaturthi begins at 9 a.m. on one day and ends at 7 a.m. the following day. By the sunrise rule, the first day is still counted as the previous tithi (Tritiya) even though Chaturthi has started, and the second day is Chaturthi even though it ends two hours after sunrise.

Now suppose the observance is one tied to the night, or to midday. The same astronomy may point to the first day instead, because that is when Chaturthi actually held at the moment that matters. That is the whole mechanism behind “why is it on Tuesday in one calendar and Wednesday in another”.

The thirty tithis, and the two halves

The lunar month divides into two pakshas of fifteen tithis each. Paksha literally means a wing or side — the two halves of the month.

Shukla paksha is the bright half, from the new moon to the full moon, as the lit portion grows. Krishna paksha is the dark half, from full moon back to new. Within each, the tithis are named in order: Pratipada, Dvitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dvadashi, Trayodashi, Chaturdashi — and then the fifteenth, which is Purnima (full moon) at the end of the bright half and Amavasya (new moon) at the end of the dark half.

The names are simply the Sanskrit ordinal numbers: Pratipada is the first, Dvitiya the second, Ashtami the eighth, Ekadashi the eleventh. So a tithi is always named with its paksha — “Shukla Ekadashi”, “Krishna Ashtami” — because every number occurs twice a month.

The five groups

Muhurta texts also sort the tithis into five recurring groups, each covering every fifth tithi: Nanda (the 1st, 6th and 11th), Bhadra (2nd, 7th, 12th), Jaya (3rd, 8th, 13th), Rikta (4th, 9th, 14th) and Purna (5th, 10th, 15th). These labels are used when choosing a time for an undertaking. They describe the quality of a day for a kind of act, not anything about a person.

Where the month begins

One more convention causes a great deal of confusion. In the amanta system, used widely in southern and western India, a lunar month ends at the new moon. In the purnimanta system, common in the north, it ends at the full moon. The bright half carries the same month name in both, but the dark half is assigned to different months. The same Krishna paksha tithi can therefore be described under two different month names, a fortnight apart on paper, while referring to exactly the same day.

Which tithis observances attach to, and why

A handful of tithis carry most of the ritual calendar, and the pattern is not arbitrary.

• Chaturthi, the fourth — associated with Ganesha. The bright-half Chaturthi is his principal day; the dark-half Chaturthi is widely kept as Sankashti Chaturthi.
• Ashtami, the eighth — a quarter-phase, associated with the Devi. Krishna’s birth, Janmashtami, is also an Ashtami.
• Navami, the ninth — Rama’s birth is observed on a bright-half Navami; see Ram Navami.
• Ekadashi, the eleventh — the principal fasting tithi of the Vaishnava calendar, observed in both pakshas.
• Trayodashi and Chaturdashi — associated with Shiva, and the Krishna paksha Chaturdashi is Maha Shivaratri. More precisely, the dark-half Chaturdashi of every month is kept by many as a monthly Shivaratri, and one of them each year is the great night itself; see Maha Shivaratri. The Trayodashi evening is observed as Pradosha.
• Purnima and Amavasya — the two extremes, and the two most widely observed. Amavasya is widely associated with offerings to ancestors, and Diwali’s Lakshmi Puja falls on an Amavasya; see Diwali and Lakshmi Puja.

The quarter, half and full points of the lunar cycle carry the weight. That is an observational calendar, built by people watching the sky rather than reading a table.

These associations are the widely shared ones, not a complete or uniform list. Regional calendars and individual sampradayas add their own, and they sometimes apply different rules to the same tithi. Ekadashi is the familiar example: Vaishnava and Smarta calendars can list it on consecutive days, because they differ over how to treat a day on which the tenth tithi still holds in the early morning.

What AdiVeda does and does not publish

AdiVeda explains what a tithi is. It does not calculate or publish tithi dates, festival dates or muhurtas.

The reason is honesty about precision. Which tithi governs a given day depends on the observer’s location and on the rule set in use, and a wrong date on a fasting day is a real failure, not a rounding error. Panchangs also differ in method: some compute positions with modern astronomical tables, others follow the traditional Surya Siddhanta computations, and the two can place the end of a tithi at noticeably different times. Where a date matters to you, take it from a panchang you trust and that states its conventions.

Where this comes from

The tithi definition and the thirty-fold division are set out in the Surya Siddhanta and the classical jyotisha and calendrical literature, and are standard across every school. The observance associations are living tradition and vary by region and sampradaya; where they do, this page says so rather than picking one.

Read on AdiVeda

Related: what a panchang is, where the tithi is the first of five limbs, and how to read a nakshatra — the nakshatra being a second, independent way the same Moon is tracked. Where the tithi measures the Moon against the Sun, the nakshatra measures it against the stars. For the story the tradition tells about Ganesha and the Moon, connected with his Chaturthi, see The Night the Moon Was Humbled.