What is Xuanwu in China mythology?

In ancient China, the stars in the sky were divided into seven star zones, namely "Three Walls" and "Four Elephants". The so-called "wall" is the "city wall". "Three Walls" is the "Wei Zi Wall", symbolizing the palace; "Taiweiyuan" symbolizes the administrative organization; The "sky wall" symbolizes the bustling market. These three walls are arranged in a triangle around Polaris. There are four elephants around the three walls: East Black Dragon, West White Tiger, South Suzaku and North Xuanwu. That is to say, the stars in the east are like dragons, the stars in the west are like tigers, the stars in the south are like big birds, and the stars in the north are like turtles and snakes. As the earth revolves around the sun, the stars in the sky change with the seasons. Every evening at the turn of winter and spring, the black dragon appears; At the turn of spring and summer, Xuanwu rose; At the turn of summer and autumn, white tigers emerge; At the turn of autumn and winter, Suzaku rises.

Xuanwu originally meant Xuanming, and the ancient sounds of Wu and Ming are interlinked. Xuan means black; Ghost means yin. At first, Xuan Ming described the tortoise divination as follows: the tortoise's back is black, and the tortoise divination is to let the tortoise go to the underworld to ask his ancestors, bring back the answers and show them to the world in the form of divination. Therefore, the earliest Xuanwu is the tortoise. Since then, the meaning of Xuanming has been expanding. Turtles live in rivers, lakes and seas (including turtles), so Xuan Ming became a water god; The tortoise lives long, and Xuan Ming has become a symbol of immortality; It turned out that the underworld was in the north, and the divination of Oracle Bone Inscriptions in Shang Dynasty was' its divination will be in the north', so Xuan Ming became a northern god again.