How to calculate the lunar leap month? How to determine leap month?

Every 19 tropic year, 7 leap months must be added, that is, "19 year 7 leap method". Which month is a leap month depends on the 24 solar terms in a year.

In the China lunar calendar, 24 solar terms are divided into 12 solar terms and 12 neutral terms. Generally, there will be a month without gas every two years, which coincides with the year when leap months are needed. Therefore, the lunar calendar stipulates that non-neutral months are leap months.

For example, on April 30th this year, the lunar calendar in China is full of vitality, and the next summer solstice is on the third day of every other month. If there is no gas this month, it will be a leap month, after April, so it is called leap April. Many people are looking for a formula to change the lunar calendar, but because of the unstable and irregular rotation of the moon, there is no formula to find. This is also the reason why China had to change its calendar every hundred years in ancient times.