In ancient and modern China, there are generally three kinds of calendar systems. The first kind is called "阴历" (yīn lì, lunar calendar), which is based on rules of the phases of the moon (new moon, first quarter, full moon and the last quarter); the second is "阳历" (yáng lì, solar calendar), which is decided upon the cycle of the earth's rotation around the sun; the third kind is the combination of solar and lunar calendar, which tells a month, decided by the moon's one-week revolution around the earth, and a year, decided by the earth's one-week revolution around the sun. This, then, is similar to "月相" (yuè xiàng, the phases of the moon), in which "初一" (chū yī, the first day of lunar January) is called "朔" (shuò, new moon), and "十五" (shí wǔ, the 15th day of the lunar month) is "望" (wàng, a full moon). This shares similar characteristics with the four seasons. So farmers have been able to arrange well their farm activities based on this rule.
The 24 solar terms (节气, jié qi), based on the rules of weather changes, are the generalization of the Han people in their farm activities. During ancient times, the Han people could tell the length of days and nights in a year and decide what the solar terms by measuring sun shadows at daytime and the location of seven Big Dippers (北斗星, běi dǒu xīng) at night. History recorded that in Spring and Autumn Period (春秋, chūn qiū, 770 B. C. - 476 B.C.), the terms "春分" (chūn fēn, the Spring Equinox), "秋分" (qiū fēn, the Autumn Equinox), "夏至" (xià zhì, the Summer Solstice) and "冬至" (dōng zhì, the Winter Solstice) had already been decided, and during the period of Warring States (战国, zhàn guó, 475 B. C. - 221 B. C.), four more solar terms had been added. They were "立春" (lì chūn, the Beginning of Spring), "立夏" (lì xià, the Beginning of Summer), "立秋" (lì qiū, the Beginning of Autumn) and "立冬" (lì dōng, the Beginning of Winter). During the times of Western Han of the Han Dynasty, 24 solar terms had already been recorded as they are today.
The 24 solar terms are in fact the result of the earth's rotation around the sun and the 24 different locations that the earth arrives at. When the earth is at different locations, there is different sunlight from the sun and so are the differences between day and night, and so come the change of the seasons due to the length of sunlight shot on earth. For example, when the sun shines directly on the earth's equator around March 21st every year, then comes the solar term "春分," on which there is the equal length between daytime and nighttime.
However, as the sun shines northwards, night becomes shorter. Around June 21st every year, the sun shines directly on the Tropic of Cancer, which makes the day the longest during the year in the Northern Hemisphere, and this makes the solar term "夏至." Then the sunlight begins to move southwards, and around September 23rd, the sunlight goes back to the equator again, which makes the solar term "秋分," on which daytime and nighttime are of the equal length again. As the sunlight continues to move southwards, days are shorter but nights are longer in the Northern Hemisphere. Around 22nd December, the sunlight shines directly on the Tropic of Capricorn, which makes the solar term "冬至" in the Northern Hemisphere, and this is the shortest daytime in a year.
For better agricultural production, farmers used to arrange their farm work according to the workings of solar terms. For example, in the south of China, "清明下种，谷雨插秧。" (qīng míng xià zhǒng, gú yǔ chā yāng. Sowing seed in Qingming, and transplanting in Guyu.) was a popular saying among farmers. In the north, however, the popular saying was "白露早，寒露迟，秋分种麦正当时。" (bái lù zǎo, hán lù chí, qiū fēn zhǒng mài zhèng dāng shí. When planting wheat, Qiufen is the right time, because the time of Bailu is too early but, when Hanlu comes it is too late). Farmers, based on their experience, have already known how agricultural work should be better arranged according to characteristics of different solar terms.
(1) "立春" (lì chūn, the Beginning of Spring), "立" means the beginning of all things on earth. "立春," then, suggests the start of the spring season, which usually falls on 3rd - 5th of February.
(2) "雨水" (yú shuǐ, Rain Water) on 18th - 0th of February, implies the coming of more raining time.
(3) "惊蛰" (jīng zhé, the Waking of Insects), "蛰" means "deeply buried." Animals, usually insects deep in the earth for winter-sleep, are called "入蛰" (rù zhé). That animals come out from the earth the next spring is believed by the ancients to be woken up by the spring thunder. Thus the time is called “惊蛰,” the start of thunder, which is usually on March 5th to 7th.
(4) "春分" (chūn fēn, the Spring Equinox) implies the equal length of daytime and nighttime (日夜平分, rì yè píng fēn). It also means that the spring season has been split into two parts. The term is usually on 20th - 22nd of March.
(5) "清明" (qīng míng, Pure Brightness), 4th - 6th of April, tells that the weather is getting warmer, grass and plants are turning green, a picture of new life, instead of the scene of dried and yellow grass and plants in cold winter, is starting.
(6) "谷雨" (gú yǔ, Grain Rain), 19th - 21st of April, tells that rainfall is increasing. Rainwater helps the grain grow. Ancient Chinese would say "雨生百谷" (yǔ shēng bǎi gǔ, rainwater helps promote the growth of hundreds of grain).
(7) "立夏" (lì xià, the Beginning of Summer), 5th - 7th of May, means the end of Spring and the start of Summer.
(8) "小满" (xiǎo mǎn, Lesser Fullness of Grain) implies that plant seeds (fruits) are getting full but are not ripening yet. The term is usually on 20th - 22nd of May.
(9) "芒种" (máng zhǒng, Grain in Beard), 5th - 7th of June, means that plants with beard-like wheat or barley are ripening and are ready for harvesting.
(10) "夏至" (xià zhì, the Summer Solstice) means that hot summer is coming. “夏至” is usually on the 21st - 22nd of June, and the daytime of that day is the longest in a year.
(11) "小暑" (xiǎo shǔ, lesser Heat), 6th - 8th of July, means the days are getting hotter. “暑” means “heat.”
(12) "大暑" (dà shǔ, Greater Heat), 22nd - 24th of July, is the hottest period in a year.
(13) "立秋" (lì qiū, the Beginning of Autumn), 7th - 9th of August, means the end of summer and the start of autumn.
(14) "处暑" (chú shǔ, the End of Heat) implies that the hot summer is coming to an end. “处” means the “end.” The term is usually on 22nd - 24th of August.
(15) "白露" (bái lù, White Dew), 7th - 9th of September, refers to the white dew condensed during nighttime when the temperature gets lower start appearing.
(16) "秋分" (qiū fēn, the Autumn Equinox) implies the equal length of daytime and nighttime. It also means that Autumn, already been split into two parts, lies between “立秋” and “立冬.” The term is usually on 22nd and 24th of September.
(17) "寒露" (hán lù, Cold Dew), 8th - 10th of October, means that temperature is getting lower and more dew appears.
(18) "霜降" (shuāng jiàng, Frost’s Descent), 23rd - 24th of October, implies the emergence of white frost due to the on-the-increase cold weather.
(19) "立冬" (lì dōng, the Beginning of Winter), 7th - 8th of November, means the end of Autumn and the start of winter.
(20) "小雪" (xiǎo xuě, Lesser Snow), 22nd - 23rd of November, means the start of snowfall because of the cold weather.
(21) "大雪" (dà xuě, Greater Snow), 6th - 8th of December, means great snow and more accumulated snow on the ground are easily seen.
(22) "冬至" (dōng zhì, the Winter Solstice) implies the arrival of the coldest time in winter. The term also tells that daytime on that day is the shortest in a year. The term is usually on 21st - 23rd of December.
(23) "小寒" (xiǎo hán, Lesser Cold), 5th - 7th of January, tells the start of the coldest season in a year.
(24) "大寒" (dà hán, Greater Cold), 20th - 21st, means the coldest time of a year.