6 The 24 Solar Terms and the Chinese Calendar

Astronomy and mathematics have always been important to the ancient Chinese. They accumulated an impressive store of knowledge about the universe, and used it to explain its existence and events. In the Chinese calendar, there are 24 Jieqi (specific stages) in a year in which there are minor climatic changes (“minor”in relation to its preceding or following stage). Each of those stages is marked by a term on a single day, indicating what the change is. For example, one of the 24 Jieqi is called Dongzhi (Winter Solstice in solar terms), on that day, the daylight hours are always the shortest in the year. The Winter Solstice is usually on the 22nd of December in the Western calendar too.

In fact, the 24 Jieqi are drawn according to the position of the earth at various times during its revolution around the sun. A revolution around the sun by the earth is a 360 degrees' turn, dividing it by 24 equates to 15 degrees, and so each stage is marked by a 15 degree turn along the path of revolution. The establishment of the 24 Jieqi was hugely beneficial to agricultural activities, and so they appeared in the Chinese calendar from very early times. As a year in the Western calendar is also the time taken for the Earth to go around the sun once, the 24 Jieqi could match the Western calendar as well.

The names of the four seasons Chun, Xia, Qiu and Dong (Spring, Summer, Autumn and Winter respectively in English) first appeared in China about 3,000 years ago. The ancient Chinese started identifying Dongzhi and Xiazhi (Summer Solstice), by standing a bamboo stick upright, and then measuring the length of its shadow at noon. They observed that the length of the shadow was the shortest on Summer Solstice, and that the daylight hours were the longest, which was the opposite for Winter Solstice. Upon more observation, it was discovered that there were 2 days in which the daytime was exactly as long as night-time; these two days were in the middle of Spring and Autumn, so the 2 days were named as Chunfen (Spring Equinox) and Qiufen (Autumn Equinox). Later, Lichun (The Beginning of Spring), Lixia (The Beginning of Summer), Liqiu (The Beginning of Autumn) and Lidong (The Beginning of Winter) were added, the 8 days marked Spring, Summer, Autumn and Winter. In the Western Han Dynasty (206 BC- 23 AD), the 8 stages were further developed into the 24 Jieqi, and they have been maintained till now.

The four seasons

As China is huge, climatic changes differ in different regions. The 24 Jieqi were measured in the area at the lower end of the Yellow River, so naturally they reflected the climatic changes of that region. The latitudes and the geographical landscape are different in other parts of China, so it is less accurate when applied to other regions. Nevertheless, it was a proof of the Chinese people's high intelligence back then.

In ancient times, whenever a new dynasty was established in place of the previous one, a new calendar would be announced to indicate the beginning of a new era. Finally in the Yuan Dynasty (1271 - 1368), a major breakthrough in the development of the Chinese calendar came in the form of Shoushi Li created by Guo Shoujing (1231 - 1316). At that time, the emperor appointed Guo, who was an expert in astronomy, to compile a new calendar. He believed that for the calendar to be reliable, the measuring apparatus must be very advanced, so he designed some new apparatus and improved the old ones, and he set up 27 observatory stations around the country. After carrying out a full-scaled investigation and detailed calculation, Shoushi Li was created successfully in the year 1280. It is also the most accurate calendar, as it adapted the advantages of other calendars, and was created with scientific calculations and theories backing it up. According to Shoushi Li, a year is 365.2425 days long and that is exactly the same as the calendar we are now using, except that it was introduced 300 years before the Gregorian Calendar -- the calendar currently in use.

Different kinds of calendars have been used in different parts of the world at different times. The three most important ones were the solar calendar, lunar calendar and a combination of both. The solar calendar considers the time taken for the earth to complete a revolution around the sun as a year, and is the most widely-used calendar in the modern world. In this calendar, there is an intercalary month once every 4 years where there is an extra day added in February. The lunar calendar considers the time taken for the moon to complete a revolution around the earth as a month and was used by Muslims -- but these days, only a minority of the Arab world still use the Islamic Calendar. The Chinese mostly used the calendar which combined both the solar calendar and lunar calendar. As it was first created in the Xia Dynasty (21st century BC - 16th century BC), it was also called Xia Calendar (or Xia Li). As farmers followed this calendar, it was also called Agricultural Calendar (or Nong Li). There are 354 days in a year in this calendar, meaning there is a deviation of 11 days compared to the most accurate calculation of 365 days a year, so in every 19 years, 7 intercalary months are added in order to maintain its accuracy. When we look at a Chinese calendar nowadays, it is common to notice that the solar calendar and the agricultural calendar are both shown, which means that the agricultural calendar is still widely observed and followed.