Yin is night; Yang is day. The earth, tethered to its gravitational rope, swings with measured purpose around the sun as does the moon around the earth. Every minute, every second, daylight-blazing across the globe–steps nimbly around the dark tumbleweed of night.
Yang is the positive side of the metaphor representing anything that is ascending or uplifting, moving forward, bright or sunny. Yin is the opposite–the negative–that which is descending or backward moving, degenerative, dark or gloomy.
Yang is summer. The days grow long. The belly of the earth warms to its perennial purpose until the axe of autumn splits summer away from the encroaching winter. The days begin to shorten. Yin is winter. The corn is gathered up; the hay is stacked in the barn.
Yin and Yang–conjoined at the hip with a hyphen–define each other by being all that the other is not: light and dark, hot and cold, top and bottom, masculine and feminine. One completes the other. In fact, one cannot be without the other. Positives and negatives attract; their codependence illustrating the delicate universal principle of balance and mutual restraint, without which everything falls apart.
Traditional Chinese Medicine views human life as an active physiological process. When the body is well and healthy, Yin and Yang are in balance; the zang-fu organs are functioning at a physiological equilibrium. When there is either an excess or deficiency on either side of the Yin-Yang equation that equilibrium is broken. Disease and illness take hold.