Tithi is a lunar day determined by the angular distance between the Sun and Moon. There are 30 tithis in a lunar month — 15 in the waxing phase (Shukla Paksha) from new moon to full moon, and 15 in the waning phase (Krishna Paksha) from full moon to new moon. Each tithi spans approximately 12 degrees of the Moon's orbit relative to the Sun. Key tithis include Purnima (full moon, the 15th tithi of Shukla Paksha), Amavasya (new moon, the 15th tithi of Krishna Paksha), and Ekadashi (the 11th tithi, sacred for fasting and worship of Lord Vishnu). Tithis vary in duration from 19 to 26 hours because the Moon's speed is not constant. The tithi at sunrise determines the tithi for the entire day in most Hindu traditions.