Cheomseongdae Observatory, constructed during the reign of Queen Seondeok (r. 632-647), is one of the landmark of Gyeongju. The observatory was built in a cylinder shape at approximately 9 meters in x_height. The observatory consists of 365 stones, symbolizing the number of days in a year. The rocks are piled in 27 layers symbolizing the 27th ruler, Queen Seondeok, and the days in a lunar month by adding the of two rock layers on top.