Longest day is a turning point

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According to historical documents, Ji Dan, the Duke of Zhou, a famous politician of the Western Zhou Dynasty, established a sundial platform in Dengfeng, which at the time was considered to be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.