Astronomers took note of "guest stars", usually supernovas or comets, which appear among the fixed stars. The supernova which created the Crab Nebula, now known as SN 1054, is an example of an astronomical event observed by Ancient Chinese astronomers. Ancient astronomical records of phenomena like comets and supernovae are sometimes used in modern astronomical studies. Cosmology The Chinese developed three cosmological models: Gai Tian ("canopy heaven") – The sky is a hemisphere, the Earth is a disc at the bottom, surrounded by water, which rotates around the North Pole once a day. The Sun traces a circle in the hemisphere, the size of which varies with the seasons. Described in the Zhoubi Suanjing. Hun Tian ("the entire sky") – Similar to Gai Tian, but the sky is a full sphere. The seasons are explained by the North Pole shifting rather than remaining directly overhead. Shuen Ye or Xuan Ye – The heavens are infinite in extent and the celestial bodies are floating about at rare intervals, and "the speed of the luminaries depends on their individual natures, which shows they are not attached to anything." Constellations The divisions of the sky began with the Northern Dipper and the 28 mansions. In 1977, a lacquer box was excavated from the tomb of Yi, the marquis of Zeng, in Suixian, Hubei Province. Names of the 28 lunar mansions were found on the cover of the box, proving that the use of this classification system was made before 433 BCE. As lunar mansions have such an ancient origin, the meanings of most of their names have become obscure. Contributing to later confusion, the name of each lunar mansion consists of only one Chinese word, the meaning of which could vary at different times in history. The meanings of the names are still under discussion. Besides the 28 lunar mansions, most constellations are based on the works of Shi Shen-fu and Gan De, who were astrologists during the period of Warring States (481 BCE – 221 BCE) in China. In the late period of the Ming dynasty, the agricultural scientist and mathematician Xu Guangqi (1562 – 1633 CE) introduced 23 additional constellations near to the Celestial South Pole, which are based on star catalogues from the West (see Matteo Ricci). Star catalogues and maps Star catalogues In the fourth century BCE, the two Chinese astronomers responsible for the earliest information going into the