The division of yin and yang in mountains and rivers is that the south side of the mountain and the north side of the water are called yang; the north side of the mountain and the south side of the water are called yin (northern hemisphere). The specific reason for its formation is that the mountain is a towering part of the ground. The sun rises in the east and sets in the west. This causes the sun's rays to shine on the south side of the mountain, but not on the north side. Therefore, the south side of the mountain is yang, while the north side is yin.
And rivers are affected by Coriolis acceleration, and objects moving horizontally in the northern hemisphere will deflect to the right (caused by the earth's movement from west to east). When the river flows, it will shift to the right bank. Most rivers in China flow from west to east. Therefore, the south bank of the river is easily impacted and eroded by the river water, resulting in a situation where the south is wet and the north is dry. Therefore, the north of the water is yang and the south is yin. .
Extended information:
The relationship between direction and superiority in ancient times:
1. In ancient times, south was regarded as the supreme, while north was symbolized as failure and surrender. When you become the emperor, you are said to be "honored by the south"; when you lose a battle or submit to others, you are "defeated" and "proclaimed by the north".
2. From the east to the west, the ancients still put the east first and the west second. The residences of the queen and concubines are divided into the East Palace and the West Palace. The East Palace is the largest and the West Palace is the second. The Ancestral Temple, where the ancestors' tablets are enshrined, should be built on the east side of the palace.
3. There are also distinctions between front, back, left, and right. The ancient emperor was the supreme king. He sat facing south and facing north, with the east on his left. Therefore, while advocating the East, "Left" also became noble. During the Three Kingdoms period, Soochow occupied Jiangdong, also known as Jiangzuo. The etiquette system of civil and military left and right, the concept of male left and female right, etc. are all reflections of respecting the left.
Baidu Encyclopedia - Landscape Yin and Yang
Baidu Encyclopedia - Direction