The Yin/Yang symbol is derived from the movement of the earth relative to the sun. When observing the cycle of the Sun, ancient Chinese used a pole posted at a right angle to the ground and recorded positions of the shadow. They divided the year’s cycle into a circle 24 Segments of about 15 days, and used six concentric circles, and recorded the length of shadow.

Beginning at the summer Solstice when the shadow was shortest and yin begins and then every 15 days, they plotted the shadow length from the outside of the circle inward until winter Solstice when the shadow was longest and Yang begins. Then they then began plotting the length from the center of the circle outward until the summer Solstice. This is because they believe the Chi energy changes directions right after the Solstice. When they connected all the measurements it resulted in the creation the familiar pattern of the Yin Yang. The Yin Yang would be the opposite color scheme if we were to record them from the southern hemisphere and this pattern would be less dramatic if one were to measure it on the equator. (link) (link)