Cheomseongdae Observatory (경주 첨성대)

Cheomseongdae is the oldest surviving astronomical observatory in East Asia, built in the 7th century during the reign of Queen Seondeok of Silla (632–647 CE) in the Silla capital of Gyeongju. The structure is a stone tower about 9.17 meters tall, built from 365 stone blocks (one for each day of the year) arranged in 27 layers (corresponding to Queen Seondeok's reign as the 27th monarch of Silla), with 12 base stones representing the 12 months of the year and 4 cardinal-direction window orientations. The observatory was used for astronomical observations to support agricultural calendar planning and political decision-making. Cheomseongdae is designated as National Treasure No. 31 of South Korea and is one of the most-visited monuments in the broader UNESCO Gyeongju Historic Areas. Pairs naturally with the Daereungwon Tomb Complex and the Gyeongju National Museum.