The reason for leap in 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 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 law, it will leap in July 2006, May 2009, April 20 12, September 20 14, June 20 17 and April 2020.