There are eight different natural phenomena which are represented by the eight trigrams. They are organized in two different ways, the pre-heaven model and the post-heaven model.
The pre-heaven, 前天 is the unconditioned, celestial state, distinguished by oneness and the true consciousness, when the original spirit is present. I consider this comparable to a perfect world where just pure function resides. The original spirit is present and perfect.
The post-heaven 后天 is after the interaction and transformation of heaven and earth. This represents the functionality of yang and yin. The post-heaven is the conditioned, human state, distinguished by discriminating consciousness, when the original spirit is hidden behind human activity.
Post-heaven is after the interaction and transformation of heaven and earth has occurred. This represents the form of yang and yin. I consider this comparable to a chaotic world that manifests from the interaction of heaven and earth. The original spirit is still present but harder to see.
In the post-heaven the original sprit or consciousness is hidden behind, acquired conditioning, compulsive habits, wandering thoughts, and the seven emotions. When the celestial yang is overcome by the mundane yin, reality is obscured. The reality is that heaven and earth are always present.
In the pre-heaven Qian heaven represents how the yang causes change.
In the pre-heaven Kun earth represents how the yin grows and changes.
In the post-heaven Qian heaven becomes fire but its function never changes. Fire arises due to the interaction of Qian heaven and Kun earth.
In the post-heaven Kun earth becomes water but its function never changes. Water arises due to the interaction of Qian heaven and Kun earth.
The compass is divided into four orthodox directions and four angles.
In the pre-heaven configuration Qian heaven is placed in the south and Kun earth in the north, and Kan water in the west and Li flame in the east.
In the post-heaven configuration Qian and Kun move to an oblique corner, but Kan and Li vault from east and west to north and south.
In the pre-heaven pattern, Kan and Li occupy the latitudinal poles, and in the post heaven pattern, Kan and Li occupy the longitudinal poles. This illustrates how important Li flame and Kan water are. The two trigrams that are the most prominent in the Classic of Changes are Kan water and Li flame.
In the post-heaven configuration, once we have transitioned from form to function, where do Qian and Kun move. They move to the oblique corners. The oblique corners, northwest and southeast, are a step down from the cardinal directions. The six children, the myriad of things all emerge from them. As soon as the six children of Qian heaven and Kun earth are born, as soon as water and fire take over, they retire from their positions.