Ying and Yang (with reflected moon overlay).
In Chinese philosophy, yin and yang (/jɪn/ and /jɑːŋ, jæŋ/; Chinese: 陰陽 yīnyáng, lit. “dark-bright”, “negative-positive”) describes how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world, and how they may give rise to each other as they interrelate to one another. In Chinese cosmology, the universe creates itself out of a primary chaos of material energy, organized into the cycles of Yin and Yang and formed into objects and lives. Yin is the receptive and Yang the active principle, seen in all forms of change and difference such as the annual cycle (winter and summer), the landscape (north-facing shade and south-facing brightness), sexual coupling (female and male), the formation of both men and women as characters, and sociopolitical history (disorder and order).
Yin and yang are better viewed as parts of an oneness that is expressed in the Tao. The term 'dualistic-monism' or dialectical monism has been coined in an attempt to express this fruitful paradox of simultaneous unity/duality. Yin and yang can be thought of as complementary (rather than opposing) forces that interact to form a dynamic system in which the whole is greater than the assembled parts.
A reflection of the moon is overlaid in part due to its textured resemblance to the Yin-Yang symbol, part to the moon's historic and mythological duality with the sun – of which this wheel of the year calendar primarily focuses upon, and also due to the fact that our moon has played a critical role in the evolution of life on Earth.