How can the lunar calendar be divided into big progress and small progress?

The period of the moon's revolution around the earth is 29.5306 days, so the number of days in the lunar calendar is 29 or 30 days (called small progress and big progress respectively).

The length of the calendar month in the lunar calendar is based on the first month of the lunar calendar, with 30 days in the first month and 29 days in the second month. The first lunar month and the second lunar month complement each other, making the average length of calendar months close to the first lunar month.

Lunar New Moon is the first day of the month-the first day of the month. Astronomically speaking, the so-called "new moon" has a definite moment, that is, the ecliptic of the moon is the same as that of the sun.

Lunar month:

The lunar calendar determines the moon on the basis of the motion law of the lunar sphere: the orbit of the moon is called the white road, and both the white road and the ecliptic are two great circles on the celestial body, which intersect at five degrees and nine minutes. The moon circled the earth once and appeared twice on the ecliptic, which lasted seven hours, 43 minutes and 11.5 seconds (27.32 days).

The time required for the moon to make one revolution is called "sidereal moon". Only when the moon revolves around the earth, the position of the earth changes because of its revolution around the sun, moving forward more than 27 degrees, while the moon moves 13 degrees 15 minutes every day.

Therefore, the 29th moon 12: 44: 2.8 (29.53 days) is called "Shuowangyue", which is called the moon by custom, meaning Shuowangyue.

The calculation of the ecliptic is very complicated, so I won't introduce it here. As for the basis for determining the name of the month in the lunar calendar, it is determined by "Zhongqi". That is, the month with rain is January.

February is a month with "vernal equinox"; The month with "Grain Rain" is March. April is a month with "small fullness"; The month with "summer solstice" is May. June is the month of "great summer heat"; July is the month of "summer heat"; The month with the autumnal equinox is August.

September is the month of "first frost"; 10 is the month of "light snow"; 1 1 month is the month of "winter solstice". 1February is the month of heavy snow. There was no leap month last month. The length of the calendar year of the lunar calendar is based on the tropic year, but the number of days in a tropic year is more than 12 moons.

However, it is shorter than 13 moons. 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 a moonless night, and the fifteenth day is a full moon, mainly with a new moon, taking into account the seasons. The method of seven leaps in nineteen years is adopted: in the nineteenth year of the lunar calendar, there are twelve average years, and one average year is twelve months; There are seven leap years and each leap year has thirteen months.