Tithi explained: the lunar day
A tithi is a lunar day: the time it takes the Moon to move 12 degrees farther from the Sun.
What it is
There are 30 tithis in a lunar month. The first 15 make the Shukla (bright) paksha, from new Moon to full Moon, and the next 15 make the Krishna (dark) paksha. The names run from Pratipada to Purnima and Amavasya.
How it is worked out or read
The tithi number is the integer part of the Moon's elongation from the Sun, divided by 12, plus one. Because the Moon moves at varying speed, a tithi can last from about 19 to about 26 hours and can be skipped or repeated on a given date.
Where it appears on this site
The tithi, with its end time, is shown in the panchang tool. See how the tithi end time is found.
Why a tithi can be skipped or repeated
A tithi is defined by the Moon's elongation, which grows at varying speed. Near perigee the Moon is fast and a tithi may be short; near apogee it is slow and a tithi may be long. In a few cases a tithi starts and ends within the same civil day, so it does not appear at sunrise at all, and the calendar skips it. In other cases a tithi covers two sunrises, and the same tithi is counted on two days. Both situations are normal and follow from the mechanics of the Moon's motion, not from an error in the almanac.
Which tithi counts for a festival
For a festival set by tithi, the rule usually asks which tithi is present at sunrise, or at midday, or at a stated time within the day. Different traditions choose different times, and the result can differ by one day. When a festival date seems wrong, check the rule the tradition follows before assuming an error. The panchang shows the tithi at sunrise and the moment it ends, which is the information needed to apply the rule yourself.
Tithi and the rest of the panchang
A tithi is one of five limbs of the panchang, and it is usually read together with the nakshatra, yoga and karana for the same day. The tithi tells you the phase of the Moon relative to the Sun, the nakshatra tells you where the Moon is among the stars, and the yoga tells you the combined motion of the two luminaries. Reading the tithi alone gives the phase but not the full picture. For this reason the panchang page shows all five together, and the reader should look at them as a set.
Tithi and the Moon's visible phase
The tithi and the visible phase of the Moon are closely linked, and a reader can check one against the other by observing the Moon in the evening sky. The tithi is the precise count; the visible phase is a useful everyday confirmation that the count matches the sky.