Cheomseongdae is an astronomical observatory in Gyeongju, South Korea. Cheomseongdae means star-gazing tower in Korean. Cheomseongdae is the oldest surviving observatory in the world. It dates to the 7th century to the time of kingdom of Silla, whose capital was Seorabeol, or present-day Gyeoungju. Cheomseongdae was designated as the country's 31st national treasure on December 20, 1962.
According to Samguk Yusa, Cheomseongdae was constructed under the reign of Queen Seondeok (632-647) near the capital of the kingdom. Cheomseongdae means "star gazing platform". The tower is built out of 362 pieces of cut granite which some claim represent the 362 days of the lunar year.  It has 27 circular layers of stones (some associate it with the fact that Queen Seondeok was considered to be the 27th ruler of Silla or the constellation of stars) 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 earth up to the level of the window. Its construction style parallels that used at the Bunhwangsa Temple in Gyeongju.
Cheomseongdae is the oldest surviving observatory in Asia.
Cheomseongdae is mentioned in popular 2009 Korean drama Queen Seondeok. In this drama, Cheomseongdae was constructed when Queen Seondeok was still a princess; this was her first decree as a princess. Cheomseongdae was meant to share the knowledge of astronomy with everyone, rather than letting one person (Lady Misil) abuse the knowledge of it. By doing so, she also abdicated her divine rights. Because this was uncommon at the time and unsupported by many conservatives, at the opening of Cheomseongdae, barely any nobles showed up.
- Indiana University Resources . Retrieved February 13th, 2006.
- Song (1983, p.17) cites the original 1962 survey of the site by Gyeongju National Museum director Hong Sa-jun, who found 366 blocks.
- Jeon, Sang-woon. (1998). A history of science in Korea. Seoul:Jimoondang. ISBN 89-88095-11-1
- Nha, Il-seong. Silla's Cheomseongdae. (2001) Korea Journal 41(4), 269-281 (2001)
- Song, Sang-Yong. A brief history of the study of the Ch'ŏmsŏng-dae in Kyongju; Korea Journal 23(8), 16-21 (1983)