In ancient China, the gnomon was used to measure the shadow of a sundial. The day with the longest shadow throughout the year was defined as "Rizhi" (also known as Ri Changzhi, Changzhi, and Dongzhi), while the day with the shortest shadow was called "Riduanchi" (or Duanzhi, Xiazhi). In spring and autumn, there were two days when the length of daytime and nighttime were equal, which were designated as "Chunfen" and "Qiufen". During the Shang Dynasty, there were only four solar terms, which increased to eight by the Zhou Dynasty. By the Qin and Han Dynasties, the 24 solar terms had been fully established. In 104 BC, Deng Ping and others formulated the "Taichu Calendar", officially setting the astronomical positions for the 24 solar terms in the calendar.
The names of the 24 solar terms first appeared in the "Huainanzi: Astronomy Training", and were also mentioned in the "Records of the Grand Historian: Taishi Gong's Preface" under the section "Discussion of the Essentials of Six Schools", referring to concepts such as yin-yang, four seasons, eight positions, twelve degrees, and 24 solar terms. During the reign of Emperor Wu of the Han Dynasty, Luo Xiahong included the solar terms in the "Taichu Calendar" and stipulated that any month without a mid-term would be considered an intercalary month of the previous month.
Each of the 24 solar terms corresponds to a specific position where the sun moves 15° along the ecliptic. The 24 solar terms are divided into 12 terms and 12 mid-terms, alternating one by one. The 24 solar terms reflect the annual apparent motion of the sun, so their dates are relatively fixed in the Gregorian calendar. In the first half of the year, the terms fall on the 6th day, and the mid-terms on the 21st; in the second half of the year, the terms occur on the 8th day, and the mid-terms on the 23rd, with no more than 1-2 days difference between them.
The naming of the 24 solar terms reflects three aspects: seasons, climatic phenomena, and climate changes. Those reflecting the seasons include Lichun, Chunfen, Li Xia, Xiazhi, Liqiu, Qiufen, Lidong, and Dongzhi, also known as the eight positions. Those reflecting climatic phenomena include Jingzhe, Qingming, Xiaoman, and Mangzhong. Those reflecting climate changes include Yushui, Guyu, Xiaoshu, Dashu, Chushu, Bailu, Hanlu, Shuangjiang, Xiaxue, Daxue, Xiahan, and Dahan.