A tithi is the Vedic lunar day — defined not by clock time but by the angular separation between Sun and Moon. Each 12° of Sun-Moon separation constitutes one tithi. The lunar month has 30 tithis: 15 in the waxing fortnight (Shukla Paksha, from new moon to full moon) and 15 in the waning fortnight (Krishna Paksha, from full moon back to new moon). Tithis are numbered 1 through 15 in each paksha, with Purnima (full moon) and Amavasya (new moon) as the 15th tithis of their respective fortnights. Each tithi has a ruling deity, an element, and specific indications for muhurta selection.