Science Hidden in365 days- 24 Solar Terms
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Scientific Common Sense
Science Hidden in365 days- 24 Solar Terms
Cheoseo, end of heat has passed and Chubun, autumnal equinox is coming. Curiously, starting around Cheoseo, the heat gradually dies down and people get ready to welcome autumn with its high blue sky. After Chubun, the nights get longer, and Hallo, cold dew comes. At last, a cool breeze blows softly. Then, autumn sets in.
Whenever we feel temperature changes little by little according to the 24 solar terms, we wonder how our ancestors created such standards for estimating the weather. Actually, our ancestors used 24 solar terms starting from a very long time ago. They divided one year into four seasons ― spring, summer, autumn, and winter ― and divided each season into six terms to make indicators for doing farming. They sowed seeds, planted rice, harvested, and prepared kimchi for the winter according to each seasonal division. The 24 solar terms were indispensable indicators in their agricultural society. Then, how were such "24 solar terms" created?
Once upon a time, when people farmed by looking at the moon
A long time ago, Korean ancestors farmed by looking at the moon. This was because the easiest way to count days from the earliest times was by using visible natural phenomena. Farmers cultivated fields by guessing the days from the change of the crescent moon and dark moon to a full moon. However, there was a problem, because the monthly cycle is average 29.53 days, not exactly 30 days. If the cycle repeated that way, it was only a matter of time before the days our ancestors expected to be warm spring became the period of heavy snow.
So, to address such a problem, they created 24 solar terms where both the solar and the lunar calendars were combined. As solar movement repeats by about 360 days, they found 15 days by dividing 360 days by 24 and regarded the 15 days as one joint period. In other words, although they guessed the days according to the shape of the moon by applying the lunar calendar, they devised one more calendar, the so-called farming-customized calendar, moving on a basis of 15 days. So, our ancestors chose a new way to understand days by the moon and do farming according to the designated solar terms.
The 24 terms, based on the length of a day, were made into a calendar that was optimized for farming. They could forecast farming for the year according to the period based on the flow of the nature. However, the 24 solar terms were not made from natural phenomena alone. The secret of the terms is also hidden from the view in astronomy, which is not visible on the land.
Following the path of the Sun, which rotates all year round
The secret lies with the Sun. Just as the earth revolves around the sun and rotates on its axis, the sun rotates on its axis, too. The path the sun takes across the sky over one year is called the ecliptic. Our ancestors found 15° by dividing 360°, which was one cycle of the ecliptic, by 24, and named each 15°, the point by which the sun moved. Ipchun, beginning of spring, Usu, rain water, Gyeongchip, awakening of insects, Chunbun, vernal equinox, etc. were the 24 solar terms created. Therefore, when the sun passes those points, the solar terms change. In Haji, midsummer, around June 21, it’s summer in the northern hemisphere, while it’s winter in the southern hemisphere. On the contrary, in Dongji, midwinter, around December 22, it’s winter in the northern hemisphere, while it’s summer in the southern hemisphere. As such, these solar terms are not applicable to all countries.
However, the solar terms are a smart calendar that reflects changes in all things according to the seasons and climate characteristics, as well as divides seasons. Chunbun, Chubun, Haji and Dongji are indicators to let us know changes in seasons, while Soseo, minor heat, Daeseo, major heat, Cheoseo, and Sohan, minor cold indicate changes in heat and cold. In particular, Soman, grain filling out and Mangjong, grain in ear, which tell us the maturity of crops and time for sowing seeds, respectively, and Gyeongchip and Cheongmyeong, clear and bright, which mean changes in nature, are all very helpful standards for farming.
As the 24 solar terms were made according to the climate state of Northern China during the Zhou dynasty, do not fit Korea’s climate. Also, the weather and seasons vary depending on various environments such as temperature and precipitation, and there is temperature rise caused by global warming in modern times. Therefore, it is difficult to follow the traditional explanation for weather in each seasonal division. However, you may wonder whether there is anything that can explain the characteristics of the seasons traditionally and somewhat romantically better than Gyeongchip, when all things wake up from their long winter sleep, and Gogu, grain rain, when spring rain falls and all kinds of crops ripen.
It’s like if Hallo comes, cold dew falls and autumn sets in our daily life.