I found this website with a nice graphic and written explanation of the white and black halves of the Yin/Yang symbol. The white half on top representing the sun, and the black half on the bottom representing the moon.
http://www.chinesefortunecalendar.com/yinyang.htm
"In general, the Yin Yang symbol is a Chinese representation of the entire celestial phenomenon. It contains the cycle of Sun, four seasons, 24-Segment Chi, the foundation of the I-Ching and the Chinese calendar."
~Mistry(Brenda)