Why does the solar calendar have 30 days for the big moon and 29 days for the small moon?

Because it takes 1 day (29.53059 days a month) for the lunar calendar to orbit the earth once, it cannot be set to 0.53059 days, so it is set to 30 days for the big month, 29 days for the small month (no 3 1 day), and 354 days or 355 days for the whole year.

The month is big and the month is small, and it is different from year to year. Since the average number of days per year is about 1 1 day less than the solar year (solar year), seven leap months are set in 19. The years with leap months are 13 months, 383 or 384 days, and leap months are called leap months after a certain month. According to the position of the sun, a solar year is divided into 24 solar terms to facilitate farming. The chronology is matched with heavenly stems and earthly branches, with a period of 60 years. According to legend, this calendar originated in the Xia Dynasty, so it is also called the Xia calendar. ?

The lunar calendar has 30 days for the big month and 29 days for the small month, and there is no month of 3 1 day. In the lunar calendar, the time for the moon to orbit the earth once (29.53059 days) is 1 month, which is the so-called "fixed value" in your question; The lunar calendar is different every year, which means that the moon is not fixed, depending on the new moon and the moon.

Extended data:

Commonly used terms:

Zodiac: the coordinate values used to determine the position of celestial bodies on the celestial sphere in the ecliptic coordinate system. * * * divided into 360 degrees. In this system, the celestial sphere is divided into two hemispheres by the ecliptic plane. The sun runs at 0 degrees of the ecliptic for the vernal equinox, at 90 degrees of the ecliptic for the summer solstice, at 180 degrees for the autumn equinox, and at 270 degrees for the winter solstice.

Homogeneous level: the uneven motion of the sun on the ecliptic is not considered in the calculation, and the first anniversary (tropic year) is divided into 24 solar terms, and the average number of days between the two solar terms is 15.22 days.

Steady gas: the uneven motion of the sun on the ecliptic is considered in the calculation. Take spring as the starting point of 0 degrees, and every time the sun runs on the ecliptic 15 degrees as a solar term, and each solar term has different time. The days between the two solar terms are different, the short one is only 14 days, and the long one is 16 days.

Solar terms: A year is divided into 12 solar terms, 12 neutral terms and * * * 24 solar terms. The solar terms were originally determined by the direction of bucket handle, and then for a period of time, they evolved into the method of "calming the qi" and then the method of "fixing the qi". Before Li Shixian in the Qing Dynasty (AD 1645), the gas was calibrated, and after Li Shixian in the Qing Dynasty, the gas was calibrated. Pay attention to the difference between labeling and calculation.

Shuori: The day when the moon moves between the earth and the sun is called Shuori.

Pingshuo: calculated by the average length of the moon, regardless of the inequality between the sun and the moon; Disadvantages: Days with real "new moons" may sometimes appear on the last day of the previous month ("dark days", that is, the 29th or 30th day of the lunar calendar), and sometimes appear on the second day.

Deciding the new moon: the inequality between the sun and the moon is taken into account in the calculation, and the day of the real "new moon" is taken as the beginning of each month.

Winter solstice: the day when the winter solstice is handed over is called winter solstice. The winter solstice must be in the second month: set the day of the winter solstice or the day before the winter solstice as the "starting point" of the first day of the second month (note: the starting point is different from fortune telling).

Calendar: it can be divided into solar calendar, lunar calendar and yin-yang calendar;

Gregorian calendar: A year is divided into 12 months, which has nothing to do with the movement of the moon. According to the date of the solar calendar, we can know that the four seasons change from cold to summer, and guide agriculture reasonably. Gregorian calendar (scientific name Gregory calendar) is a kind of solar calendar.

Lunar calendar (lunar calendar): The lunar calendar in China does not belong to this category, but people are used to calling it the lunar calendar in life. Take the waning period of the moon (new moon, equal to 29.530588 days) as one month, and the product of 12 months is one year. The annual apparent motion law of the sun is completely ignored, so the date of the lunar calendar can't show the warmth and coldness of the four seasons. This calendar has no practical value, except for Islam, which has been abandoned.

Lunar calendar: China lunar calendar belongs to this category. It takes the moon phase, that is, the change cycle of the moon, as the month length and the return year of the sun as the age. By setting leap months, the average calendar year adapts to the tropic year. According to the date of the lunar calendar, we can not only know the ebb and flow of tides, but also grasp the changes of seasons.

Almanac: A publication that records calendars and other related information, such as yellow calendars and perpetual calendars. Electronic almanac is the product of scientific and technological development and a new carrier.