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There is a day that is celebrated, without prior agreement, in many countries of the world, considered special by people who observe the movement of the sun. On its eve, in England, people rally to Stonehenge, a mystical ring of monoliths erected by ancient devotees, to welcome rise of the sun. In Latvia, people travel from the cities to the countryside to gather, eat and sing in celebration.
Chinese people attach equal importance to this day, as the Summer Solstice, or xiazhi in Chinese, which falls on Tuesday this year, is the tenth of the traditional Twenty-Four Solar Terms.
In the Northern Hemisphere, Summer Solstice occurs when the sun is directly over the Tropic of Cancer. It is the longest day of the year in terms of daylight. After the solstice, the subsolar point begins its migration south, and daytime gradually shrinks.
Despite what many might think, with it being at the height of summer, xiazhi is not the hottest time of year. A Chinese folk saying goes: "The summer is not hot enough until Summer Solstice".
After xiazhi, the temperature will continue to increase. Just like Winter Solstice is start of "nine nine"-a system to record the following 81 days-in winter, this year, Tuesday marks the beginning of the "nine nine" of summer.
Chinese people used to count the days in periods of nine, and then summarize the conditions during each period in songs. One example of a summer nine nine song describes the increasingly warmer weather following the Summer Solstice, before it finally starts to turn cold.
"In the first and second nine, you always need a fan in the hand. In the third nine, you can find ice water as sweet as honey. Your clothes are always soaked with sweat in the fourth nine. You can feel gentle wind blow the branch in the fifth nine. The sixth nine is a good time to relax in a cool place. During the seventh nine you need to be covered with quilt when sleeping. Don't catch a cold in the eighth nine. Everybody finds their coat in the ninth nine."
Ancient Chinese believe the Summer Solstice is the time when yang energy is at its most potent, and gets weaker as the days progress, giving way to yin energy, which increases in strength. That is to say, Summer Solstice is the turning point between yin and yang, while Winter Solstice marks the reverse of that energy flow.
As a result, typical phenomena of xiazhi include deer-which symbolize yang energy-shedding their antlers as yin energy starts to grow. Also because of the rise of yin, cicadas start to chirp, and people start hearing them more often. Banxia, an herb strong in yin energy, begins to grow, marking a point when living things with yin energy begin to thrive, while those strong in yang energy gradually wither.
According to Lyu Hongjun, a scholar of the Twenty-Four Solar Terms, xiazhi is very important since it is one of the first solar terms.
"China's Twenty-Four Solar Terms didn't appear all at once, but developed over a long period. In the Western Zhou Dynasty (c.11th century-771 BC) the two solstices and equinoxes appeared. Then in the Spring and Autumn Period (770-476 BC), the start of the four seasons were added to the system. During the Warring States Period (475-221 BC) the rudiment of the Twenty-Four Solar Terms was basically established, and their names were decided across the Qin (221-206 BC) and Han (206 BC-AD 220) dynasties," Lyu says.
In Dengfeng city, Central China's Henan province, two ancient structures tell the story of the observation of the sun by ancient Chinese, and the origin of the Twenty-Four Solar Terms.
According to historical documents, Ji Dan, the Duke of Zhou, a famous politician of the Western Zhou Dynasty, established a sundial platform in Dengfeng, which at the time was considered to be the center of the earth, to observe the movement of the sun.