A Celestial Kingdom on Earth
In Gyeongju, the ancient capital of the Silla Kingdom, the area contains dozens of enormous grassy mounds. These are the royal tombs of the Silla dynasty, which lasted nearly a millennium (57 BCE–935 CE). Located within the Daereungwon Tomb Complex, these tumuli are outwardly simple, manicured hills rising from the flat city center, but their construction was a major feat of engineering. A typical tomb consists of a wooden chamber for the deceased and their burial goods, which was then sealed and covered with a massive mound of river stones, and finally capped with a thick layer of earth. This design made the tombs largely impenetrable, and preserved their contents for centuries.
The most famous of these tombs include Hwangnam Daechong, the largest, and Cheonmachong, the "Heavenly Horse Tomb." Cheonmachong, excavated in 1973, is the only tomb interior currently open to the public. Inside, visitors can see replicas of the treasures found within, including a brilliant gold crown and a saddle flap painted with a mythical winged horse, which gave the tomb its name. While large, painted star maps on ceilings are more characteristic of the earlier Goguryeo Kingdom to the north, Silla’s interest in the cosmos is clear. Their astronomical knowledge was advanced, and celestial motifs appear on many artifacts recovered from these very tombs.
Forensic Astronomy in the Silla Court
The rulers of Silla viewed astronomy as a state-level science, essential for agriculture and political stability. Their most famous astronomical achievement, the Cheomseongdae Observatory, is just a short walk from the Daereungwon tombs. Built during the reign of Queen Seondeok (632–647 CE), it is the oldest surviving astronomical observatory in East Asia. The bottle-shaped stone tower, 9.17 meters high, was constructed with 362 granite blocks, representing the days of the lunar year. Its 27 layers of stone are thought to represent Queen Seondeok as the 27th ruler of Silla.
This deep astronomical knowledge provides the context for dating a king's death using a star chart found in a tomb. The science of this practice relies on the precession of the equinoxes—the slow, gradual wobble of Earth on its axis. This wobble causes the apparent positions of stars to shift over centuries in a predictable way. By creating a precise model of the 7th-century night sky, astronomers can match the specific stellar alignments depicted on an ancient map to a very narrow window of time. If a star map from a royal tomb ceiling was detailed enough, its unique celestial snapshot could date the tomb, allowing scientists to calculate the year—and perhaps even the season—of its creation.