An astronomical observatory in Gyeongju, South Korea. Cheomseongdae is the oldest surviving astronomical observatory in Asia, and possibly even the world. It was constructed in the 7th century in the kingdom of Silla, whose capital was Seorabeol, or present-day Gyeongju.

Cheomseongdae was designated as the country's 31st national treasure on December 20, 1962. It forms a UNESCO World Heritage Site along with other Gyeongju Historic Areas.

The building itself represented knowledge; the number of stones represents days of the year (scholars differ on whether it contains 362 or 365 large stones representing days in the solar or lunar year). The stones appear in 27 courses (for Seondeok, the 27th ruler) with 12 courses above and below the window entrance for the months of the year and these sum to 24, the number of solar terms in a year (24 points in traditional East Asian lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon). Including the stylobate, or platform on which it was built, gives 28 courses and 28 symbolizes the 28 constellations of East Asia. The addition of the two-tier top brings us to 30, the number of days in a month. The tower itself is a gnomon of a sundial and the window captures the sun’s rays on the interior floor at spring and autumn equinoxes. Astronomy was of vital importance as it governed agriculture and contemporary scientists produced detailed star charts. Astrology influenced political decisions of the day. Thus observations at Cheomseongdae were of utmost importance to Silla.
→Ancient Korean Astronomer Queen - Queen Seondeok of Silla