A Calendar in Stone
Cheomseongdae, which translates to "star-gazing tower," is a physical calendar built of granite. Its construction encodes specific astronomical and cultural values of the 7th-century Silla Kingdom. The tower stands approximately 9 meters high and is composed of 27 circular layers of stone blocks. This number is believed to represent Queen Seondeok, the 27th ruler of Silla, who commissioned the observatory's construction around 633 CE.
The total number of granite blocks used is subject to some debate among scholars, but it is often cited as 362 or 365, representing the days in a lunar or solar year. The structure itself is a combination of shapes. It rests on a square, two-tiered base, which represents the four seasons. The bottle-shaped, cylindrical body shows the roundness of the heavens. Halfway up the structure, a square window opens to the south. There are 12 layers of stone both above and below this window, representing the 12 months of the year and the 24 solar terms of the traditional East Asian calendar.
Silla's Celestial Watchtower
Astronomers of the Silla Kingdom used Cheomseongdae to observe the sky, track the movements of stars, and forecast weather. These observations were essential for agriculture, determining the correct times for planting and harvesting. Astronomy was also closely linked to politics and astrology; celestial events were signs of the king's mandate from heaven.
The observatory's design allowed scientific observation. The south-facing window was positioned to capture sunlight on the interior floor during the spring and autumn equinoxes. Observers would have placed a ladder on the outside to enter through the central window. From there, another ladder could be used to reach a platform at the top, where instruments may have been placed. The lower half of the tower is filled with earth and rubble, which gives the structure a low center of gravity and has helped it withstand earthquakes and weather for nearly 1,400 years. Records show that after Cheomseongdae was built, documented celestial observations in Korea increased significantly.