Cheomseongdae (Hanja: 瞻星臺) is the oldest existing astronomical observatory in Asia. Constructed during the reign of 27th Monarch of Silla Dynasty, Queen Seondeok (Reigned: 632-647) - it was used for observing the stars in order to forecast the weather. This stone structure is a beautiful combination of straight lines and curves, and was designated as Korean Republic National Treasure No.31 on December 20th, 1962. It is located at Inwang-dong 839-1 beonji, Gyeongju City, Northern Gyeongsang Province.
According to Samguk Yusa, Cheomseongdae was constructed under the reign of Queen Seondeok near the capital of the kingdom. Cheomseongdae means "star gazing platform". The tower is built out of 362 pieces of cut granite which some claim represent the 362 days of the lunar year. Some surveys of the site have indicated that there are 366 blocks. It has 27 circular layers of stones (some associate it with the fact that Queen Seondeok was considered to be the 27th ruler of Silla or the constellation of stars) surmounted by a square structure. 12 of the layers are below the window level and 12 are above. There are 12 large base stones set in a square, with three stones on each side. These sets of 12 may symbolize the months of the year.
The observatory was built in a cylinder shape with stones 30cm in diameter. 362 stones were piled up to make 27 levels. Roughly 4.16m up from the bottom there is 1 sq m square entrance and a space to hang a ladder under it. Its construction style parallels that used at the Bunhwangsa Temple in Gyeongju.
The inside is filled with soil up to the 12th level, and the 19th, 20th, 25th, and 26th levels all have long rocks hanging on two areas, shaped as the Chinese letter '井' (jeong).
It stands 9.17m high and the base stone on each side measures 5.35m. The Vernal Equinox, Autumnal Equinox, Winter Solstice, Summer Solstice and the 24 solar terms (also known as the astronomical solar year) were determined by the observation of stars. The pavilion stone is believed to have been used as a standard of deciding directions, north, south, east and west. The 362 stones used to build Cheomseongdae represented the 362 days in a lunar year.
Cheomseongdae is the oldest surviving observatory in East Asia, though some claim that it was not suitable for astronomical observation. Others posit that it was used for astrology rather than astronomy, though during that era there was little differentiation between the two. Some scholars have argued that Cheomseongdae's design was heavily influenced by Buddhism, noting similarities between its design and Mount Sumeru, the center of the world according to Buddhist mythology.