Toponymy
Many places in China, such as Luoyang, contain the word "Yang," and a few, such as Huayin, contain the word "yin." This is a very old way to assign place names.
Classically, when used in place names, "yang" refers to the "sunny side." The word 太陽 (simplified 太阳) tàiyáng, refers to the sun, and literally means "great yang." In the northern hemisphere, sunlight comes predominantly from the south, and so the south face of a mountain (or the north face of a river valley) will get more direct sunlight. Therefore, "Yang" means a place is on the south slope of a mountain (or on the north bank of a river valley). For example, Luoyang is on the north bank of the Luo River Valley.
In the same way, "yin" would be the opposite, the "shadowy side." "Yin" means that a place is on the north slope of a mountain (or on the south bank of a river). For example, Huayin is on the north slope of Mount Hua.
Read more about this topic: Yin And Yang