Tithi
every reader
What the tradition reads
A tithi is a lunar day, defined not by a fixed length of clock time but by the angular distance between the Sun and the Moon increasing by exactly twelve degrees, so that a tithi's actual duration varies from about twenty to twenty seven hours depending on how quickly or slowly the Moon happens to be moving in its elliptical orbit at that point in the month. A full lunar month contains thirty tithis, split into two fortnights of fifteen each: the shukla paksha, the bright or waxing half running from new Moon to full, and the krishna paksha, the dark or waning half running from full Moon back to new, each fortnight numbered pratipada through chaturdashi before closing on either purnima, the full Moon, or amavasya, the new Moon. Whichever tithi is current at sunrise is treated as the tithi of that whole day for calendrical purposes, even though the tithi itself may end, or a second tithi may begin and end, before the following sunrise, an irregularity the tradition names and tracks as kshaya, a lost tithi, or adhika, a doubled one. The tithi is one of the five limbs of the panchanga, the traditional Hindu almanac that also tracks the vara, the nakshatra, the yoga and the karana for any given day, each limb measured by its own angular relationship between the Sun and Moon rather than by the clock, tithi's own irregular length simply the most visible example of a pattern the whole panchanga shares.
The story
Tithi is a Sanskrit word meaning specifically a lunar day, one of the various Sanskrit time units, alongside the muhurta and the nakshatra, that the tradition developed to measure time by the Moon's own motion rather than by the more even, solar based hours the calendar otherwise runs on. The definition by degrees of elongation rather than by any fixed span of hours reflects an old and precise piece of observational astronomy, since a tithi measured this way tracks the Moon's actual angular progress away from the Sun exactly, at the cost of the day's length itself becoming irregular in a way a purely calendrical unit would not be. The system of thirty tithis divided into two fortnights of waxing and waning remains the backbone of Hindu religious observance as much as of astrological practice, since festival dates across the Hindu calendar are set by tithi rather than by the solar date, which is why the same festival can fall on a different solar calendar date from one year to the next.
Written from
Corpus key glossary:tithi.