In Chinese philosophy the yin and yang are generalized descriptions of the antitheses of the nature.

The early Han dynasty (207 B.C.-9 A.D.) devoted itself to regaining the same level of central government as the Ch'in (221 B.C.-207 B.C.) and the Legalists had so ruthlessly accomplished. This ideology of central government, along with the Legalists' attempts to standardize Chinese culture and Chinese philosophy, led thinkers of the Han to attempt to unify all the rival schools of Chinese thought and philosophy that had developed over the previous three hundred years.

The Legalists attempted to standardize Chinese thought by burning the books of rival schools and by making it a capital crime to speak of Confucius, Lao Tzu, or Mo Tzu. The Han thinkers, who thoroughly despised the Legalists and their methods while adopting many of their goals, took a different approach; rather than reject alternate ways of thinking, they took a syncretic approach and attempted to fuse all the rival schools of thought into a single system. This syncretic project of the early Han is known as the Han synthesis.

The Han philosophers concentrated specifically on the Five Classics, attempting to derive from them, particularly the I ching , or Book of Changes, the principle of the workings of the universe, or Tao. This new theory of the universe they appended to the I ching ; this appendix explains the metaphysical workings of the entire universe and is the origin of what is called the yin-yang or Five Agents school of Chinese thought.

The yin and yang represent all the opposite principles one finds in the universe. Under yang are the principles of maleness, the sun, creation, heat, light, Heaven, dominance, and so on, and under yin are the principles of femaleness, the moon, completion, cold, darkness, material forms, submission, and so on. Each of these opposites produce the other: Heaven creates the ideas of things under yang, the earth produces their material forms under yin, and vice versa; creation occurs under the principle of yang, the completion of the created thing occurs under yin, and vice versa, and so on. This production of yin from yang and yang from yin occurs cyclically and constantly, so that no one principle continually dominates the other or determines the other. All opposites that one experiences—health and sickness, wealth and poverty, power and submission—can be explained in reference to the temporary dominance of one principle over the other. Since no one principle dominates eternally, that means that all conditions are subject to change into their opposites.

Where does the Yin Yang Symbol derive from?
Accent Chinese people by observing the sky, recording the Dipper's positions and watching the shadow of the Sun from an 8-foot (Chinese measurement) pole, ancient Chinese determined the four directions. The direction of sunrise is the East; the direction of sunset is the West; the direction of the shortest shadow is the South and the direction of the longest shadow is the North. At night, the direction of the Polaris star is the North.

They noticed the seasonal changes. When the Dipper points to the East, it's spring; when the Dipper points to the South, it's summer; when the Dipper points to the West, it's fall; when the Dipper points to the North, it's winter.

When observing the cycle of the Sun, ancient Chinese simply used a pole about 8 feet long, posted at right angles to the ground and recorded positions of the shadow. Then they found the length of a year is around 365.25 days. They even divided the year's cycle into 24 Segments, including the Vernal Equinox, Autumnal Equinox, Summer Solstice and Winter Solstice, using the sunrise and Dipper positions. They used six concentric circles, marked the 24-Segment points, divided the circles into 24 sectors and recorded the length of shadow every day. The shortest shadow is found on the day of Summer Solstice. The longest shadow is found on the day of Winter Solstice. After connecting each lines and dimming Yin Part from Summer Solstice to Winter Solstice, the Sun chart looks like below. The ecliptic angle 23 26' 19'' of the Earth can be seen in this chart.

By rotating the Sun chart and positioning the Winter Solstice at the bottom, it will look like this The light color area which indicates more sunlight is called Yang (Sun). The dark color area has less sunlight (more moonlight) and is called Yin (Moon). Yang is like man. Yin is like woman. Yang wouldn't grow without Yin. Yin couldn't give birth without Yang. Yin is born (begins) at Summer Solstice and Yang is born (begins) at Winter Solstice. Therefore one little circle Yin is marked on the Summer Solstice position. Another little circle Yang is marked on the Winter Solstice position. These two little circles look like two fish eyes.