Tithi
A lunar day — the time it takes the Moon to gain 12 degrees on the Sun.
A tithi is a lunar day, defined by the Moon moving 12 degrees ahead of the Sun. There are thirty tithis in a lunar month: fifteen in the waxing fortnight (shukla paksha) ending at the full moon, and fifteen in the waning fortnight (krishna paksha) ending at the new moon.
Because the Moon's speed varies, a tithi is not a fixed 24 hours — it can be a little shorter or longer than a solar day, so tithis can occasionally repeat or be skipped on the calendar. The tithi running at sunrise usually names the day.
Tithis carry traditional qualities and govern festivals and fasts: Ekadashi (the 11th), Purnima (full moon) and Amavasya (new moon) are well known. The tithi is the first of the Panchanga's five limbs.
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