Chinese star names are named according to ancient Chinese astronomy and astrology. The sky is divided into star mansions and asterisms. The ecliptic is divided into four sectors that are associated with the Four Symbols, guardians in Chinese mythology, and further into 28 mansions. Stars around the north celestial pole are grouped into three enclosures. The system of 283 asterisms under the Three Enclosures and Twenty-Eight Mansions was established by Chen Zhuo of the Three Kingdoms period, who synthesized ancient constellations and the asterisms created by early astronomers Shi Shen, Gan De and Wuxian. Since the Han and Jin dynasties, stars have been given reference numbers within their asterisms in a system similar to the Bayer or Flamsteed designations, so that individual stars can be identified. For example, Deneb is referred to as 天津四.