In case you ever wondered where the Yin-Yang symbol comes from, see my paper entitled A Geomedical Approach to Chinese Medicine: The Origin of the Yin-Yang Symbol, which is a chapter in a book on the recent advances in Chinese Medicine. Most other books and articles deal mostly with the philosophical aspects of the Yin-Yang symbol and could not care less about its origin. In fact, the true origin is widely unknown, even among Chinese people. This is a pity because the origin of the Yin-Yang symbol is less of a philosophical nature and more of a technical nature. Knowing the origin of the Yin-Yang symbol helps to understand that the roots of Chinese philosophy are quite accessible to analytical methods. In my paper, I present a mathematical description of the Yin-Yang symbol based on the daily sunshine hours throughout the year. I will write more about the Yin-Yang symbol here in due course. For the time being, this is the reference to the paper:
Stefan Jaeger (2011). A Geomedical Approach to Chinese Medicine: The Origin of the Yin-Yang Symbol, Recent Advances in Theories and Practice of Chinese Medicine, Prof. Kuang Haixue, Heilongjiang University of Chinese Medicine, China (Ed.), ISBN: 978-953-307-903-5, InTech, Available from: http://www.intechopen.com/articles/show/title/a-geomedical-approach-to-chinese-medicine-the-origin-of-the-yin-yang-symbol
More information, including nice illustrations, is available on Allen Tsai’s webpage.