Tithi

A Tithi is defined as the time it takes for the Moon to gain 12° of longitude over the Sun. Since the Moon moves approximately 13.2° per day and the Sun about 1° per day, the relative angular gain is about 12° per day — hence roughly one Tithi per day. However, because the Moon's orbital speed varies (perigee vs. apogee), a Tithi can last between 19 to 26 hours.