Tithiतिथि

A tithi is the time the Moon takes to gain 12° of longitude on the Sun. Thirty of these steps complete one synodic (new-moon to new-moon) month. Because the Moon’s speed varies across its orbit, a tithi is not a fixed clock interval — it ranges from roughly 19 to 26 hours, which is why a tithi can occasionally span two sunrises or be skipped between them.

The thirty tithis split into two fortnights (paksha). The fifteen of Shukla Paksha run from the new moon to the full moon (Purnima); the fifteen of Krishna Paksha run from the full moon back to the new moon (Amavasya). Particular tithis carry traditional observances — the eleventh of each fortnight is Ekadashi, the fifteenth of the bright fortnight is Purnima, and the last of the dark fortnight is Amavasya.

SahiKundli computes the tithi from sidereal Sun and Moon positions: the elongation is (Moon longitude − Sun longitude), reduced to the 0–360° range, and the tithi number is that value divided by 12. The tithi reported for a date is the one in effect at sunrise; its end time is the moment the elongation next reaches a multiple of 12°, located by bisection on the live positions and shown in the location’s timezone. Reporting the boundary, rather than a single “today’s tithi” label, keeps the variable length of the lunar day honest.