By observing the cycle of the Sun, we can use a pole, post at right angles to the ground and record the lengths of the shadow about every 15 days for a year. The shortest shadow is found on the day of Summer Solstice in China. The longest shadow is found on the day of Winter Solstice. The day of Winter Solstice has the least sunshine in the year. After Winter Solstice, the day will gain more sunshine each day till Summer Solstice. We can say Yang begins right after Winter Solstice and Yin begins right after Summer Solstice in the northern hemisphere.
After recording 24 shadow lines, we move the lines into the diagram of six concentric circles with 24 sectors beginning from Winter Solstice to Summer Solstice. The length of each line will subtract the portion of the length of Summer Solstice shadow line because we want to give Summer Solstice maximum Yang.
If the shadow of the pole is observed at equator, then the length of the shadow on Summer Solstice should be near 0. China is in the northern hemisphere. The shadow of the pole always shows there on Summer Solstice at any city of China. To adjust the shadow to zero length on Summer Solstice is to adjust latitude to the equator.
From Winter Solstice to Summer Solstice, the shadow lines are drawn from the center of the circle to the boundary. From Summer Solstice to Winter Solstice, the shadow lines are drawn from circle boundary to the center, because ancient Chinese believed that Chi Energy goes different directions right after Summer Solstice and Winter Solstice.
Next, we can connect all the lines and dim the shadow lines (Yin) part, the Ying Yang symbol appears. .
© Copyright 1999- Allen Tsai
Dr. Stefan Jaeger presented a mathematical description of the Yin-Yang symbol based on the daily sunshine hours throughout the year in the A Geomedical Approach to Chinese Medicine: The Origin of the Yin-Yang Symbol in 2011.