What is the reason for leap in April?

Twenty-four solar terms are calculated according to beginning of spring, odd numbers are called "solar terms" (beginning of spring, Jing Zhe, Qingming, Long Summer, Mangzhong, Xiaoshu, beginning of autumn, Bailu, Cold Dew, beginning of winter, Heavy Snow and Slight Cold), and even numbers are called "Zhongqi" (Rain, Equinox, Grain Rain, Xiaoman, Summer Solstice, Big Tree, Early Tree and Autumn Equinox).

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 it runs very fast at the perihelion (vernal equinox and autumnal equinox), and the maximum 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 (Xinsi year of the snake), Xiaoman is May 2 1 (April 29th of the lunar calendar), and the first day of the next lunar month is May 23rd. However, this lunar month ends on June 20, and the summer solstice falls on June 2 1 day, so this lunar month is not "neutral", but in. According to this leap rule, it will leap in July 2006, May 2009, April 20 12, September 20 14, June 20 17, April 2020 and February 2023.