Things are yin or yang depending on the criteria you apply. We can't say that something is yin or yang in itself, only in relation to something else. And something can be both yin and yang, depending on the criterion of comparison being used. A black ball, for instance, is yin in colour (relative to white, yang) but yang in shape (a sphere relative to a cube, a circle being yang and a square yin).
The chill of the valley when the sun is no longer overhead is a dynamic representation of the bright sunny midday yang energy changing later in the day to a cold yin energy and preparation for nightfall. Similarly, night is yin, day is yang, moon is yin, sun is yang. Yin and yang change into each other when they have grown 'tired' and 'old', essentially when they have reached an extremity of themselves and can no longer go on being themselves. So the system of yin and yang is one of constant change and relativity of states. The waxing and waning of the moon dynamically represents the advance of yang (and decline of yin) followed by the advance of yin (and decline of yang), which is illustrated in my notes on the bigua sequence of hexagrams.
Yin and yang are characterisations of changing states to highlight different natures and what causes one thing to change into another. Earth itself is yin and heaven yang. This is represented on a Chinese coin, the square hole in the middle is seen as yin and earth, the round circumference of the coin is yang and heaven. At first, it is best to simply accept these ideas rather than try to 'understand' them. After all, this system is not the world itself but an arbitrary overlay to understand certain things about it in simple terms. When you think of things being either 'woody' or 'tinny', 'antique' or 'reproduction', you are doing much the same thing, discerning qualities.