法苑珠林 4.83

4.83

白虎通曰。日行遲。月行疾。日行一度。月行十三度十九分度之七。日月徑千里。又計日行路。有其內外。從極北至極南。相去九百九十由旬。經一百八十日。日行從內至外。又經一百八十日。日行從外至內。是故名行。言日行六十里者。由輪大故。日遲天行。以行遲故。唯六十里。是故一年有十二月。六月北行。六月南行。總有三百六十度行路也。白虎通曰。月所以滿缺。何歸功於日也。三日成魄。八日成光。二八十六轉而歸功。晦至朔旦。受符復行也。月有大小何天左旋。日月右行。日行遲。月行疾。月及日為一月。至二十九日未及七度。即須三十日過七度。日不可分。故乍小明。有陰陽即有閏月。何周天三百六十五度四分度之一。十二月日不匝十二度。故三年一閏。五年再閏也。明陰不足陽有餘。閏者陽之餘也。

The Bohu tong says: 'The sun moves slowly, while the moon moves swiftly. The sun moves one degree, while the moon moves thirteen degrees and seven-nineteenths of a degree. The diameter of the sun and moon is 1,000 li. Moreover, calculating the path travelled by the sun, it has its inner and outer [limits]. From the extreme north to the extreme south, the distance is 990 yojanas. Passing through 180 days, the sun travels from the interior to the exterior; passing through another 180 days, the sun travels from the exterior to the interior. Therefore it is called traveling. That the sun is said to travel sixty li is because its wheel is large. The sun is slower than heaven's movement; because its travel is slow, it is only sixty li. Therefore one year has twelve months: for six months it travels north, and for six months it travels south, with a total course of 360 degrees.' The Bohu tong says: 'Why does the moon wax and wane? It returns its merit to the sun. On the third day the crescent body (po) forms; on the eighth day the light forms; at two eights—the sixteenth—it turns and returns its merit. From the last day of the month to the dawn of the new moon, it receives the tally and moves again. Why do months have long and short periods? Heaven rotates to the left, while the sun and moon travel to the right. The sun moves slowly, while the moon moves swiftly. When the moon catches up with the sun, it makes one month. At twenty-nine days, it has not yet reached it by seven degrees; it then requires thirty days to exceed it by seven degrees. The day cannot be divided, hence months are sometimes short. When there are yin and yang, there are intercalary months. Why is this? The circuit of heaven is 365 and one-quarter degrees. In twelve months the sun does not complete twelve degrees; therefore, there is an intercalary month every three years, and two intercalary months in five years. This clarifies that yin is deficient while yang has a surplus; the intercalary month is the surplus of yang.'