The cause of leap March

In fact, the lunar calendar is a kind of "combined calendar of yin and yang", which is the same as the lunar calendar and similar to the solar calendar. It is based on the moon's profit and loss cycle as the calendar month, and also takes into account the annual solar terms.

Generally speaking, the moon's profit and loss period is 29.53 days (29 days 12 hours, 44 minutes and 2.9 seconds), which is called "Wang Shuo Moon". The big moon has 30 days, the small moon has 29 days, and a year is 12 months * * 354 days. The lunar month has no seasonal significance. In this way, the twelve lunar months constitute the lunar year, with a length of 29.5306× 65,438+02 = 354.3672 days, which is 65,438+less than the 365.2422 days in the tropic year. The difference between solar calendar year and year is 1 1 day. Only after 17 will the date of the lunar calendar be reversed in the same season. For example, a year's New Year is spent in the snow, and a new year comes after 17. The use of such a calendar naturally cannot meet the needs of agricultural production, so the lunar calendar in China has been in parallel with the 24 solar terms since the Qin and Han Dynasties, and the 24 solar terms are used to guide agricultural production.

In order to synchronize the lunar calendar with the solar calendar in seasons, our ancestors adopted the method of "adding leap months" to "adjust the surplus and deficiency" and set seven leap months in 19, because the total number of days in the tropical year of 19 is basically equal to 19 plus seven leap months in the normal year. Therefore, the lunar calendar stipulates a leap month every two or three years. In the year with leap month, the lunar calendar is 13 months, accounting for 383 days.

So, in which month should this leap month be placed?

Through long-term practice, our ancestors divided a year into 24 solar terms, which were placed in 12 months, and each month had two fixed solar terms: the former was called "Festival" and the latter was called "Zhong". Such as "beginning of spring" and "Rain" in the first month. The former is "frugality" and the latter is "moderation". Solar terms and neutral gas are alternately arranged in monthly order. The average interval between two adjacent solar terms and neutral gas is 30 and a half days. However, the average lunar month (the first lunar month) is only 29 and a half days. In this way, next month's solar terms or gas will be delayed by one or two days from the previous month's solar terms and gas. At a certain point in time, "Zhong Qi" was not in the middle of the month and then moved to the end of the month. Next month, there will be only one kind of "frugality" and no "moderation". According to the lunar calendar, any month with only solar terms and no neutral atmosphere can not be regarded as a month independently, but only as a subsidiary (sub-month) of a month-leap month.

Leap March occurs when the month after March has only solar terms, and the neutral gas after solar terms falls to another month, so there will be a month without neutral gas after March, so the month without neutral gas after March is named "leap March", which is the reason for leap March.