A calendar on the wall changes its date at midnight. A panchang does not work that way. Its most important day-count, the tithi, is set by the Moon and the Sun, and it can begin at any hour — at dawn, at noon, or in the middle of the night.
The Moon’s lead over the Sun
Every month the Moon starts level with the Sun (the new Moon, Amavasya) and slowly pulls ahead of it, until it stands opposite the Sun (the full Moon, Purnima) and then comes back again.
A tithi is simply every 12° of that lead. Twelve degrees, thirty times, makes the full circle of 360°. So a lunar month has thirty tithis:
- Shukla paksha, the bright half: Pratipada to Purnima, fifteen tithis while the Moon grows.
- Krishna paksha, the dark half: Pratipada to Amavasya, fifteen tithis while it wanes.
Why a tithi is not 24 hours long
The Moon does not move at an even speed. Near the Earth it runs faster; far away, slower. So the time it takes to gain 12° on the Sun changes all the time. A tithi can last as little as about 20 hours or as long as about 27.
That is why the tithi rarely changes at midnight, and why yesterday’s tithi may end at 9 in the morning while today’s ends at 11.
The tithi at sunrise names the day
If a tithi can change at any hour, which one does the day belong to? The tradition answers: the tithi running at sunrise (udaya tithi). The day carries that name until the next sunrise, even if the tithi itself ends at noon.
Two things follow from this:
- Sunrise matters. Sunrise in Guwahati comes almost an hour before sunrise in Ahmedabad. When a tithi changes close to dawn, one city can wake up in the old tithi and the other in the new one. Their panchang for the same date can then differ.
- Some tithis are skipped or repeated. A short tithi can begin after one sunrise and end before the next. It never touches a sunrise, so no day carries its name — a kshaya tithi. A long tithi can touch two sunrises, and two days carry its name — a vriddhi tithi.
The change happens at one instant
The moment a tithi ends is the same instant everywhere on Earth, because it depends only on the Sun and the Moon. What differs from place to place is the clock time you read it in, and the sunrise it is compared with.
How festivals use the tithi
Most festivals are tied to a tithi, but not always to the tithi at sunrise. Some look at noon, some at the evening, some at midnight. We explain these rules in why festival dates differ between cities.
In short
- A tithi is 12° of the Moon’s lead over the Sun; thirty make a lunar month.
- It lasts roughly 20 to 27 hours, so it changes at any time of day.
- The tithi at your sunrise names your day — which is why the panchang must be worked out for your own city.