According to the rotation of the earth, people have produced the phenomenon of alternating day and night, forming the concept of "day"; According to the revolution of the moon around the earth, there is also a new moon, thus forming the concept of "moon"; According to the alternation of four seasons caused by the revolution of the earth around the sun, the concept of "year" was formed. Year, month and day are a unit of time, which consists of three concepts, and the material movements on which these three concepts are based are independent of each other. According to accurate measurement, the time for the earth to circle the sun is about 365.2422 flat solar days, which is called the tropic year; The continuous time interval from one new moon to the next is 29.5306 flat solar days, which is called new moon.