A tithi is also called as lunar day. There are 15 tithi's in the waxing cycle of the moon (shukla paksha), and there are 15 tithis in the waning cycle of the Moon ( Krishna paksha). The tithi is based on a relationship between the Sun and the Moon. The first tithi is 12 degrees of the Moon away from the Sun after the new Moon (Amavasya) or full Moon (Purnima). The second tithi is the next 12 degrees of the Moon away from the Sun, 12 to 24 degrees. A particular day is ruled by the tithi at sunrise, but the tithi can change anytime of the day or night as it is not based on the solar day.