Cheomseongdae is an astronomical observatory in Gyeongju, South Korea. Cheomseongdae is the oldest surviving astronomical observatory in Asia, and possibly even the world. It was constructed during the Silla period, during the reign of Queen Seondeok. Cheomseongdae was designated as the country's 31st national treasure on December 20, 1962. It is part of Gyeongju Historic Areas, a UNESCO World Heritage Site.