A tithi is one thirtieth of a lunation, defined as twelve degrees of the Moon's elongation from the Sun: the tithi runs from the moment the elongation reaches a multiple of twelve degrees to the next multiple. There are thirty in a lunar month, fifteen waxing (shukla paksha) and fifteen waning (krishna paksha), and their names are the same in both fortnights with the fifteenth being Purnima in the bright and Amavasya in the dark.