Why is there a leap in April?

In the China lunar calendar, a year with a leap month is called a leap year. The general year is 12 months, 364 or 365 days, and the leap year is 13 months, 383 or 384 days. As a kind of lunar calendar, the number of days in each month depends on the monthly loss, and the time of one year is based on 12 months. In order to close the earth's cycle around the sun, that is, the tropic year, one month is added every two to four years, and the added month is a leap month, so the leap year of the lunar calendar is 13 months. There is no thirteenth month in the lunar calendar. According to the calendar rules, leap months are repeated from February to the same month after 10, and the repeated months are leap months. For example, the leap month after April is called leap April.

The working people in ancient times called the previous solar terms (beginning of spring, Jing Zhe, Qingming, Long Summer, Mang Zi, Xiaoshuhu, beginning of autumn, Bailu, Cold Dew, beginning of winter, Heavy Snow and Slight Cold) "solar terms". The last solar terms (rain, vernal equinox, Grain Rain, Xiaoman, Summer Solstice, Great Summer, Early Summer, Autumn Equinox, Early Frost, Light Snow, Winter Solstice and Great Cold) are called "Zhongqi". The average value between two neutral gases is 365.2422÷ 12=30.4368 days, then 30 or 3 1 day. The earth's orbit around the sun is elliptical and moves slowly at the perihelion (vernal equinox and autumnal equinox), and the interval between the two solar terms can reach 3 1.45 days. On the other hand, the average lunar new moon is only 29.5306 days, including 29 days in the first lunar month and 30 days in the second lunar month. In this way, there will be such a situation: in some months (usually abortion), the last breath has passed at the beginning, and the next breath has not come at the end. For example, 200 1 (New Year's Eve) is May 21(April 29), and the first day of the next lunar month is May 23. However, this lunar month ends on June 20, and the summer solstice is June 2 1, so this lunar month is not "neutral", it falls in the fourth month of the lunar calendar. According to this leap rule, it will leap in July 2006, May 2009, April 20 12, September 20 14, June 20 17 and April 2020.