How is it calculated in the perpetual calendar?

The perpetual calendar is evolved from the calendar.

It is called "calendar" to calculate the length of year, month and day and the relationship between them, and make the law of time sequence. The almanac is a reference book for people to sort out the years and solar terms. In ancient China, the almanac was called the general book or the constitutional book. In the feudal dynasty, it was also called the "imperial calendar" because it was promulgated by the emperor.

According to the alternation of day and night caused by the rotation of the earth, people formed the concept of "day". According to the concept that the moon revolves around the earth to form a "moon" and the concept that the seasons alternate to form a "year" caused by the earth's revolution around the sun. 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 time interval from a new moon to the next consecutive appearance is 29.5306 flat solar days, which is called the new moon. Taking the tropical year as the unit, how many integer months are arranged in a year, how many integer days are arranged in a month, and how to choose the starting point of a year are all called calendars.

Calendar principle

No matter what kind of calendar, there is a problem of coordinating the relationship between calendar day period and astronomical period. In principle, the calendar month should be equal to the new moon, and the calendar year should be equal to the Tropic of Cancer. However, because the first month of the lunar calendar and the return year are not complete days, calendar months must be divided into large months and small months, and calendar years must be divided into normal and leap years.

Through the proper collocation and arrangement of big month and small month, flat year and leap year, the average calendar month is equal to the new moon, or the average calendar year is equal to the tropic year. This is the main content of the calendar.

Reference to the above content: Baidu Encyclopedia-China Calendar