What a tithi is, and why it is not a day

The definition

At the new moon the Sun and the Moon are in the same direction from the Earth: the angle between them is zero. The Moon then pulls ahead, gaining roughly twelve degrees a day. Each twelve degrees of that gain is one tithi. Thirty of them bring the Moon all the way round to the Sun again.

The first fifteen are the śukla pakṣa, the bright fortnight, ending at the full moon. The next fifteen are the kṛṣṇa pakṣa, the dark fortnight, ending at the new moon. Within each fortnight the tithis are numbered — Prathamā, Dvitīyā, Tṛtīyā, and so on to Caturdaśī — with Pūrṇimā and Amāvāsyā as the fifteenth of each.

Why the length varies

The Moon’s orbit is an ellipse. Near perigee it moves fastest, near apogee slowest, and the Sun’s apparent speed varies a little too. The rate at which the gap between them opens therefore changes through the month, and with it the length of a tithi.

Working from the actual speeds, a tithi can be as short as about twenty hours and as long as about twenty-six and three-quarters. The average is about twenty-three hours and thirty-seven minutes — a little less than a civil day, which is why most tithis are shorter than the day they sit in.

Kṣaya and adhika tithis

Because the average tithi is shorter than a day, one occasionally begins after sunrise and ends before the next sunrise. It therefore never “holds” at any sunrise, and does not get a day of its own. That is a kṣaya tithi — a lost one. In 2026 there are eighteen of them.

The opposite also happens: a long tithi can hold at two consecutive sunrises and appear twice. That is an adhika tithi.

These are not errors. They are the reason a lunar calendar and a solar one cannot be made to line up, and the reason an almanac has to be computed rather than counted.

How a day gets its tithi

A civil day is usually named for the tithi that is running at its sunrise. But not every observance follows that rule: Pradosham is judged at sunset, because the pradoṣa period is the hour and a half around dusk; Sankaṣṭī Caturthī is judged at moonrise, because the fast is broken on sight of the Moon; Masik Śivarātri is judged at niśīta, the true middle of the night.

Judging all of them at one fixed hour is a common software error, and it is the reason a calendar can disagree with the temple down the road. This site anchors each observance to the moment its own tradition defines it by.

What a family may do

• When a relative in India tells you the date of a vratam, ask which tithi it is. The tithi will be the same for both of you; the date may not.
• Before a household observance, check the tithi’s end time as well as its name.
• If a tithi ends early in the morning, ask Gurukkal how your family keeps it — traditions differ on whether the observance follows the sunrise or the span.

Often misunderstood

• That “today is Ekādaśī” means the whole day is Ekādaśī. It usually is not.
• That a missing tithi is a calendar mistake. A kṣaya tithi is ordinary astronomy.
• That the tithi number is the date. Śukla Caturthī is not the fourth of the month.

Where traditions differ

• Amānta traditions (much of the south and west) end the lunar month at the new moon; pūrṇimānta traditions (much of the north) end it at the full moon. The tithis are identical; the month names attached to them differ by a fortnight.

Explaining it to a child

Think of the Sun and the Moon walking around a huge circle. The Moon walks faster. Every time the Moon gets twelve steps ahead, that is one lunar day. Sometimes the Moon hurries and sometimes it dawdles, so the lunar days are not all the same length.