Cheomseongdae Observatory is thought to have been built during the reign of Queen Seondeok (r. 632-647) of the Silla Dynasty for observation of the movements of heavenly bodies. It is the oldest astronomical observatory in Asia and is widely regarded as a precious cultural heritage that gives us a clear idea of the level of technological development of that time.
Standing approximately 9 m in height, this observatory consists of a cylindrical body, a platform, and a square top. The cylindrical body includes 27 layers of fan-shaped stones, and the outer face is trimmed smooth whereas the interior wall face is not even. At the upper part, long ends of stone material that is geared into inside part of hash shape are extended to the outside. This arrangement can be seen in layers 19-20 and 25-26, and so it is assumed that a ladder was easily placed easy on those steps in the inside. Around the southeast opening, the lower part is filled with rubble, while the upper part to the top is open and hollow. The eastern half of the uppermost step is blocked with a stone slab. It is presumed that there might have been some devices for observation inside the observatory. The record of an ancient book states, “People can climb up through its middle.” It seems that one simply placed a ladder on its outside and climbed inside through a window, and then climbed up to the top via another ladder to observe the stars. One researcher has suggested that the top might have been very convenient to sit, stand or lie on. During the ancient period, astronomy was deeply related to agriculture as farming times were often determined according to the movements of the stars. It was also deeply related to politics in that the horoscope was considered to be important, since the good or bad fortune of a nation could be predicted according to the results of their observation of the stars.