And they name it as the shape the septuple Primary Stars.

北方七、、、虚、危、室、壁，其形如龟，称玄武。

In fact, they divided Heaven into four parts and each part is composed by septuple Primary Stars.

西方七奎、娄、胃、昂、毕、角、参形状如，称西宫。南方的井、鬼、柳、、张、翼、轸联鸟形，称朱雀。