Digging up the Earliest Astronomical Observatory in China

At the town of Taosi, the county of Xiangfen, Shanxi province the earliest (up to date about 4000 years ago) astronomical observatory and sacrificial altar relic was dug up, which consists of an observing site, some tamped soil columniations and slits between those columniations, This construction was used to observe the variations of the sunrise azimuth and determine the tropical year length in order to const itute the calendar. It is indicated from the simulated observations that the two slits located in the southeast and the northeast could be precisely used to determine the dates of the Winter Solst ice and the Summer Solstice. Between those two slits there are 10 columniations which could indicate that the visual Sun moving from the one columniation to another is a solar term. It implies that in the Emperor Yao time the calendar was the solar calendar in which one year was divided into 20 solar terms. The Yin-Yang five-element calendar, that is, 10-month solar calendar in the very ancient time was based on this calendar.