What Is a Tithi? Why Lunar Days Stretch, Shrink, Skip and Repeat

A tithi is each 12-degree step in the angle between the moon and the sun - not a day on a calendar, but a slice of the moon's monthly journey around the sky. When moon and sun sit at the same longitude, the angle is zero: amavasya, the new moon. As the moon pulls ahead, every 12 degrees it gains completes one more tithi - Pratipada, Dwitiya, Tritiya and onward - until at 180 degrees the moon stands opposite the sun at purnima. Then the angle closes again through the waning fortnight back to zero. Thirty steps of 12 degrees make the full 360-degree circle: one lunar month, two fortnights, thirty tithis. Everything puzzling about tithis - why they run long or short, why one occasionally vanishes from the calendar, why ekadashi lands on a Thursday this year and a Saturday next - follows from this one definition plus a fact of orbital mechanics.

The moon does not move at a constant speed

The moon's orbit around the earth is an ellipse, not a circle. Near perigee, its closest approach, the moon moves fastest against the stars; near apogee, its farthest point, it moves slowest. Its apparent speed swings between roughly 12 and 15 degrees per day. The sun, meanwhile, appears to crawl forward about one degree per day, so the gap between them grows at anywhere from about 11 to 14 degrees per day depending on where the moon is in its ellipse.

A tithi is 12 degrees of that gap. When the moon is racing near perigee, it covers 12 degrees of separation in as little as about 20 hours. When it is dawdling near apogee, the same 12 degrees can take nearly 27 hours. So tithis breathe: they stretch and shrink continuously through the month, and no two tithis are exactly the same length. This is not an imprecision in the panchang. It is the panchang faithfully reporting an elliptical orbit.

The sunrise rule: how a tithi gets attached to a day

Since a tithi can begin and end at any hour, the tradition needs a rule for which civil day "owns" it. The rule is udaya tithi: the tithi in effect at local sunrise gives the day its name. If Ekadashi begins at 10 p.m. tonight and ends at 8 p.m. tomorrow, then tomorrow's sunrise falls inside Ekadashi, and tomorrow is observed as the Ekadashi day - even though the tithi actually started tonight. (Some observances use a different anchor: Pradosham wants Trayodashi at dusk, Sankashti Chaturthi wants Chaturthi at moonrise, Masik Shivaratri wants Chaturdashi at midnight, and a yearly shraddha is done on the day the death tithi holds the afternoon (aparahna). That is why a purohit will occasionally tell you an observance falls a day away from what the sunrise rule alone would suggest. The tradition prescribes the anchor per observance; the astronomy just provides the timeline.)

Skipped and repeated tithis

Now combine the two facts: tithis vary from about 20 to nearly 27 hours, and days are stamped at sunrise, roughly 24 hours apart. Two mismatches become inevitable:

• Kshaya (skipped) tithi. A short tithi can begin just after one sunrise and finish just before the next. It never touches a sunrise, so it never gets a day. The calendar appears to jump - say from Chaturthi straight to Shashthi - even though Panchami happened in full; it simply happened entirely between two sunrises.
• Vriddhi (repeated) tithi. A long tithi can span two sunrises, catching both. Then two consecutive civil days carry the same tithi name, and the calendar shows, say, Saptami twice in a row.

Both are routine. A typical year contains many of each, and every classical panchang maker has handled them with the same sunrise rule for centuries.

So why is ekadashi on different days in different years?

Here is the honest answer, with no mystery left in it. The lunar month is about 29.5 days long, which does not divide evenly into weeks or civil months. So the moment the moon-sun angle reaches the Ekadashi span drifts through the civil calendar continuously - about 11 days earlier per year, until an added lunar month resets it. Which weekday it lands on is just arithmetic fallout. And because the observance is stamped at sunrise, the same Ekadashi can even attach to different civil days in different places: if the tithi ends mid-morning, a city where the sun rose before the tithi ended observes it that day, while a city with a later sunrise may have missed it. Nobody is computing the moon differently; they are reading the same sky through different sunrises.

Even in one city, two households can keep Ekadashi a day apart. The Smartha rule usually takes the day on which Ekadashi holds at sunrise. Vaishnava traditions - Madhvas, Sri Vaishnavas, ISKCON and others - also need the previous tithi, Dashami, to have ended before arunodaya, the 96 minutes before sunrise; when it has not, their fast moves to the next day. Our guide to which panchang your family follows gives an example.

Reading tithis the way they were meant to be read

A tithi is best understood as a live angle with a deadline, not a box on a grid. This is why the daily panchang always shows the tithi with its end-time - "Shukla Panchami until 14:32" - and why the tradition insists rituals honour the tithi's actual span. A shraddha, a vrat, a puja tied to a specific tithi is tied to when that 12-degree arc is genuinely in effect at your location. The panchang on Purohit.app computes that arc from planetary positions using the Swiss Ephemeris and reads it against your city's own sunrise, which is exactly what a careful panchang maker with a telescope-grade ephemeris would do by hand. The moon sets the schedule; the panchang just reports it honestly.