What is the neutral gas in solar terms?

The time when the sun runs along the longitude of the Yellow River from zero 15 degrees is called "a solar term". It runs 360 degrees every year, and * * * experiences 24 solar terms, twice a month. Among them, the first solar term of each month is "solar term", that is, beginning of spring, Jingzhe, Qingming, Changxia, Mangzhong, Xiaoshu, beginning of autumn, Bailu, Hanlu, beginning of winter, Heavy Snow, Xiaohan, etc. 12 solar terms; The second solar term of each month is "Zhongqi", that is, there are 12 solar terms such as rain, vernal equinox, Grain Rain, Xiaoman, summer solstice, great heat, intense heat, autumnal equinox, first frost, light snow, winter solstice and great cold. "Solar term" and "neutral qi" appear alternately, each lasting 15 days. Now people have called "solar terms" and "neutral gas" solar terms.

The arrangement of solar terms depends on the sun. In the Western Zhou Dynasty and the Spring and Autumn Period, the sum time of winter and summer solstice was determined by measuring the sun shadow with a standard table. After that, the length of a tropical year is divided into 24 parts. From the winter solstice, solar terms and neutral atmosphere are arranged in equal parts. This method is called flat gas. According to the method of balancing qi, every month has a solar term and a neutral qi. For example, beginning of spring is the solar term of the first month, and the rain is the qi of the first month; The earthquake is the solar term in February, and the spring is the gas in February. Because two solar terms last longer than a new moon, there may be only one solar term or a neutral atmosphere in a month. Taichu calendar in the Western Han Dynasty stipulated that solar terms could appear in the second half of last month or the first half of this month, and qi must appear in this month; If you encounter a neutral vernal equinox, you can make it a leap month last month. This principle has been used for more than a thousand years. During the Northern Qi Dynasty (550-577 AD), Zhang Zixin discovered the uniform motion of the sun. In the fourth year of Emperor Wendi's Renshou (AD 604), according to this uneven phenomenon, Liu Zhuo proposed a reform on the twenty-four solar terms and divided Sunday into twenty-four solar terms. When the sun moves to every point, it is the moment of a solar term. The interval of solar terms arranged in this way is uneven, and this method is called fixing qi. Fixed gas is mainly used for calendar calculation. Pingqi has been used in daily calendars until the Qing Dynasty. The names of the 24 solar terms are: beginning of spring, Rain, Sting, Equinox, Qingming, Grain Rain, Changxia, Xiaoman, Mangzhong, Summer Solstice, Xiaoshu, beginning of autumn, Chushu, Bailu, Equinox, first frost, Chufrost, beginning of winter, light snow, heavy snow, winter solstice, slight cold and severe cold.