As early as the Hsia Dynasty 4000 years ago, the ancient Chinese observed the shadows of the sun and discovered the summer solstice--when the sun's shadow is the shortest and the daylight lasts the longest--and the winter solstice --when the sun casts the longest shadow and daylight is least.
早在四千多年前的夏朝，先民們觀測日影，發現有日影最短、白晝最長的「夏至」和日影最長、白晝最短的「冬至」，成為後來製定節氣的濫觴。