The Zhanxingtai in Gyeongju was built in 634 AD. It is the oldest astronomical building on the Korean Peninsula and the oldest existing observation platform in Asia. The cylindrical building with a diameter of 5 . 17 hrs, a height of 9 . 4 hrs, was built with 362 granite piles representing a year. Using the light reflected by the water mirror and window at the bottom to observe the astronomical phenomena such as eclipse and star movement, the national event and farmers' harvesting operations in North Korea are determined by divination. Zhanxingtai is designated as the national treasure of the Republic of Korea 31st.