Excuse me, how is the size of the lunar month determined? What is its law?

In the lunar calendar, the months of the year are set according to the cycle of the moon phase, with the 29th of each month as the small moon and the 30th as the big moon. The length of the calendar month in the lunar calendar is based on the first month of the lunar calendar, and the date of the new moon (that is, when the moon is between the sun and the earth and the dark half faces the earth) is fixed as the first day of the month-the first day.

The time of the first month of the lunar calendar is 12 hours, 44 minutes and 3 seconds (that is, 29.53059 days), which is a little over 29 and a half days. Because the calendar must use integers, it is stipulated that the first lunar month is 30 days and the second lunar month is 29 days. The first lunar month and the second lunar month complement each other, so that the average length of calendar months is close to the first lunar month.

But it is not exactly the same as the pure lunar calendar, because the pure lunar calendar is arranged alternately in large and small months, and the size of the lunar year is determined by calculation. In addition, the moon is generally sixteen times in the big moon (looking at the sun) and fifteen times in the small moon (looking at the sun).

The size of the lunar month is very irregular, and it is difficult to find a rule like the Gregorian calendar. It is different which month is bigger and which month is smaller in a year, sometimes it is two, three, four big months or two consecutive three small months. Because the first month of the lunar calendar is slightly longer than 29 and a half days, there are about 53 big months and 47 small months in every 100 calendar month of the lunar calendar.

In most parts of China, we can observe whether there is a thin crescent in the western sky after sunset (evening) on the second day of the second lunar month to determine the size of the month. If there is a crescent moon, it means the first night of the lunar calendar, so this month is longer, but the big month is 30 days; No new moon means that the first day of the lunar new moon happens earlier, which is shorter. It's the little moon, it's the 29th.

Extended data:

The division standard of big and small months in the solar calendar

The lunar calendar is fixed with small single month, big double month and big leap twelfth month. Except mantissa 000, 165, 335, 500, 665 and 835, the first month of every multiple year of 5 is changed to big month.

333 and 1/3 years have 333.333×12+122.74386 = 4122.486 months, of which (333.333 ÷ 5)-2 = 64.486.

There are 2000+122.754386+64.666666 = 2187.4205438+053 months and 1935.3333 months.

Equivalent to 12 1747.2982 days, with an average of 29.53057273 days (29 days 12 hours and 44 minutes 1.483 seconds), with an average of 365.24 days1895 days (395 days).

The mid-year month of the lunar calendar is based on the length of 29.5306 days in the first lunar month, so the big month is 30 days and the small month is 29 days. In order to ensure that the first day of each month (the first day) must be a new moon, the arrangement of large and small months is not fixed and needs to be determined through strict observation and calculation.

Therefore, it is common for the lunar calendar to be big and small for two consecutive months, and even there are rare exceptions such as February and March of 1824. August, September, 10,1month are big for four consecutive months.

Baidu encyclopedia-lunar calendar