[Origin of the S curve in the ancient Taiji diagram-evolvement from the heaven and earth positioning diagram to the Taiji diagram].

The term 'Taiji' first appear in Zhouyi·Xici, saying that 'yi has Taiji which includes yin and yang'. Chinese ancestors formed the 'year' conception by observing sunrise, sundown and seasonal successions for a long period. 'yi has Taiji which includes yin and yang' was the resultant recording of their sun-movement observations. People compared the heaven and earth to a circle and they divided the circle into four areas to record meteorological changes of the four seasons. A line was drawn from the spring equinox to the autumn equinox and the heaven and earth positioning diagram was created with the upper half as the heaven and the bottom half as the earth. The line was called the Taiji line. Considering that 'yang starts to grow at the winter solstice and yin starts to grow at the summer solstice', therefore people gradually changed the line to a S -curve.
PMID: 22613480