《隋书·列传·卷四十三》
月道与黄道相交，在黄道内运行十三日有奇而出，又在黄道外运行十三日有奇而入，周而复始，月亮经过黄道，称为交。初一、十五在距交点十五度以内，就有月蚀现象。
月道交络黄道，每行黄道内十三日有奇而出，又行黄道外十三日有奇而入，终而复始，月经黄道，谓之交，朔望去交前后各十五度已下，即为当食。
The lunar orbit and ecliptic intersect, and the moon travels inside the ecliptic for thirteen and a fraction days, then exits and travels outside the ecliptic for thirteen and a fraction days before starting the cycle again. When the moon passes through the ecliptic, it is called an intersection. On the first and fifteenth days, if the moon is within fifteen degrees of the intersection, there will be a lunar eclipse.