The Cheomseongdae astronomical observatory (첨성대), also spelled Chomsongdae, is an astronomical observatory which was built circa 647 in Silla, one of the three kingdoms of ancient Korea. It was constructed under the rule of Queen Seondeok near Gyeongju (慶州), the capital of the kingdom.
Cheomseongdae means "star gazing tower". The observatory is built out of 362 pieces of cut granite which represent the 362 days of the lunar year. As can be seen from the photograph, it has 27 circular layers of stones (Queen Seondeok was the 27th ruler of her dynasty) 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 tower is 5.7 meters wide at the base and 9.4 meters tall, and filled with dirt up to the level of the window. Its construction style parallels that used at the Bunhwangsa temple and was likely a product of contact with the Tang Dynasty of China, with which Queen Seondeok allied her country.
Cheomseongdae is the oldest surviving observatory in its geographic region. It has been designated by the South Korean government as the country's 31st national treasure since December 20th, 1968.
