Why is there a "leap moon" in the lunar calendar?

There will be a leap month for the following reasons:

1, China Lunar New Year (New Year's Day) did not appear in summer, because China gradually formed a unique method of adjusting the lunar calendar from the Han Dynasty-"fixed leap method". The current lunar calendar method is "seven leaps in nineteen years", that is, every two to three years, a lunar month that is the same as the previous month must be added, and the added month is called a leap month.

2. The rules of leap method are based on 24 solar terms related to the tropic year of solar calendar. A tropical year is divided into 24 solar terms. If the twenty-four solar terms are discharged from beginning of spring to severe cold, then odd numbers are called solar terms and even numbers are called qi.

3. Using twelve Chinese Qi to represent the twelve months of the lunar year, the average interval between Chinese Qi and Chinese Qi will be one day closer than that of a lunar calendar. In the long run, there will always be a phenomenon of gas neutrality at the end of the month, so there will be no gas next month, only solar terms. So this lunar month without neutrality is called the leap month of last month.