Leap moon is the rarest.

Leap December is the rarest of all leap months. The twelfth lunar month is the last month of a year, whether it is a normal year or a leap year. The twelfth lunar month (leap) refers to the leap December, that is, the thirteenth month of the year is a leap month and also a twelfth lunar month. In order to keep the return year of the lunar calendar in sync with the Gregorian calendar, the ancients adopted the method of setting leap months.

Since the constitutional calendar, the new moon and qi have been fixed, and the time between them is less than 30 days at the perigee (near the slight cold), so there are fewer leap months, especially the twelfth and first months. Leap December is the rarest leap month among all leap months. From17th century to 33rd century, there was no leap lunar month (but it was not static, and the frequency of leap lunar month gradually increased from the lowest), and there was no leap lunar month in 36th-38th century, 4th1-47th century and 49th century.

The origin of leap month

Leap month is one of the ways of calendar leap, that is, 1 month added to the lunar calendar in leap year. The lunar calendar is a lunar calendar, which stipulates that a month without gas is a leap month, and the name or ordinal number of the month above is called a "leap month". When ancient astronomers compiled the lunar calendar, in order to make any day in a month contain the meaning of the moon phase, that is, the first day is moonless night and the fifteenth day is full moon.

Taking the first month of the lunar calendar as the benchmark, taking into account the seasons and seasons, the method of seven leaps in nineteen years is adopted: in the nineteenth year of the lunar calendar, there are twelve normal years and one normal year is twelve months. There are seven leap years and each leap year has thirteen months.

In Asia (especially China), leap month refers to one month added every two to three years in the lunar calendar. In order to coordinate the contradiction between the tropic year and the lunar year and prevent the lunar year from being out of touch with the tropic year, that is, the four seasons, 1 leap is set every 2~3 years. In ancient times, there was a leap week, and there were seven leaps in nineteen years. By the time of Linde calendar in Tang Dynasty, the fixed leap week was abolished, and the leap was set in the month without season, so it was leap time.