The theory that the original Yin-Yang symbol described the change of a pole’s shadow length during a year corresponds well with the meaning of “Taiji” which means “Great pole” in Chinese. “The Origin of the Yin-Yang Symbol” paper further explains the meaning of it: “The winter solstice, the beginning of the lengthening of the day, is the white dot in the middle of the black part of the diagram. It is the seed of light in the middle of the darkest part of the year. But even though the days start lengthening, the weather gets colder until the midpoint between the winter solstice and spring equinox… The summer solstice, the beginning of the shortening of the day, is the black dot in the middle of the white part of the diagram. It is the seed of darkness in the middle of the whitest (lightest, brightest) part of the year. But even though the days start shortening, the weather gets warmer until the midpoint between the summer solstice and autumn equinox… This endless cyclical change of the duration of the day and night is what is seen as the endless cyclical interplay between the light (Yang) and darkness (Yin) which heats (Yang) and cools (Yin) the Earth and creates seasons and makes life possible.”
This would be an amazing explanation for the origin of Taiji, except for one problem with it. The origin of the Yin-Yang symbol lies in the graphical representation of the daily change of a pole’s shadow length. This length varies for each day, when measured at the same time, and depends on the geographic latitude of the observer. For Latitude L = 40◦, which is about the latitude of Beijing, the daylight curve is flatter and would look like the chart below.
For Latitude L = 50◦, which is about the latitude of Kyiv and the area where the ‘Yin-Yang’ symbol was in use during the times of the Trypillian culture, the symbol currently would look like the following:
But it is important to remember that the shape of the Yin-Yang symbol also depends on the ecliptic angle of the earth. The ecliptic’s obliquity, which is the sun’s apparent path around the Earth is not stable and can change during the millennia. This is due to the different forces exerted by the bodies in the solar system on Earth. It means that in the year 5, 000 B.C., the ecliptic was not the same as now and the ancient Yin-Yang symbol looked slightly different than the modern-day Yin-Yang symbol for the same area.
At the present time, the latitude at which Yin-Yang starts looking close to what we are used to is
You may want to read about the origin of ‘Chinese’ dragons as well