法苑珠林 4.57

4.57

According to the Lokasthitābhidharma-śāstra: What is the dark half? What is the bright half? Because of the sun there is the dark half, and because of the sun there is the bright half. The sun constantly pursues the moon's movement. Day by day they approach each other by forty-eight thousand and eighty yojanas, and day by day they separate from each other by the same amount. When they approach each other, day by day the moon's disk is obscured by three yojanas and one-third of a yojana. For this reason, on the fifteenth day the moon is completely obscured; this day is the culmination of the dark half. Day by day it separates from the moon also by forty-eight thousand and eighty yojanas, and day by day the moon is unveiled by three yojanas and one-third of a yojana. For this reason, on the fifteenth day the moon is fully unveiled, pure, and complete; in the world this is called the culmination of the bright half. If the sun and moon move at their greatest distance from each other, at that time the moon is full; in the world it is spoken of as the culmination of the bright half. If the sun and moon are together in one place, this is called conjoined movement; in the world it is spoken of as the culmination of the dark half. When the sun moves behind the moon, the sun's light illuminates the moon's light; because the moon's light is coarse, when it is illuminated it casts a shadow, and this moon-shadow in turn conceals the moon itself. Therefore, the back part of the moon is seen not to be round. For this reason, it is gradually veiled until the fifteenth day when the moon is completely veiled; when [the sun] moves behind it, this is called the dark half. If the sun moves ahead of the moon, it is unveiled and purified day by day in the same way, until the fifteenth day when it is fully round; when [the sun] moves ahead of it, this is called the bright half.