Many thousands of years ago, there were no written languages in any civilization. The Chinese were no exception. People expressed themselves by looking at images and draw pictures to represent their simple thoughts and ideas. However, the Chinese paid a lot of attention to the changing phase of the moon in relation with the sun.
The sun is very bright and generates heat. It is considered Tai Yang. Tai means very as I mentioned before. The moon does not generate light nor heat. Without the sun, the moon has no light, therefore, it is always very dark. Thus, the moon is considered a Tai Yin.
The sun is bright all the time and it is always in its ultimate state. Its Yang state never changes. That is why it is called Tai Yang.
During each month in the lunar calendar, we can see the moon change from a slim crescent to a full circle and back again. The moon has two Tai Chi states. At the new moon phase and at the full moon phase. At new moon, the moon is at the extreme dark state. It is in the Tai Chi State of Yin. At full moon, the moon is at the extreme bright state. It is in the Tai Chi State of Yang. Therefore, the sunlit surface of the moon is considered Yang and the dark surface is considered Yin.
Due to the changing phase of the moon in relation with the sun, the Chinese used this relationship to indicate change. The original idea was to draw two figures by using a circle with a dot in the middle to represent the sun and a slim crescent for the moon. These two figures with the sun on the left and the moon on the right become a symbol to indicate change in the universe. Later, the Chinese derived characters from the imaged figures. The modern version of the change symbol is with the sun radical on top and the moon radical at the bottom to form one character. This character pronounced as Yi which mean change. Thus, the Yi Jing is the Classic Book of Change. Of course, Jing means classic book.