Furthermore, the shadow of the sun is at its longest on the winter solstice, and it is useful for checking the sundial to see if it is working correctly. Therefore, the early Chinese called the time between one winter solstice and the next a "Latitudinal Year." Forget the dates, remember the Terms The length of a month is based on the moon.
由於以太陽為準，才能契合四季輪替，又由於冬至日影最長，有利於校正日晷，於是先民將這個「冬至」到下一個「冬至」之間的時間，稱為一個「回歸年」。