Credited to South Korea. The tower's origin in Silla is not in question, but its function is. Since the 1960s some scholars have argued that it was an altar, modeled on the Buddhist Mount Sumi or dedicated to a god of agriculture, and not a working observatory. No instruments or observation records survive.
How it began
Cheomseongdae was constructed in Gyeongju, capital of the Silla kingdom, during the reign of Queen Seondeok, who ruled from 632 to 647. Silla occupied part of what is now South Korea. Seasonal change governed decisions about agriculture, and astrology linked observation of the sky closely to politics, so there was strong interest in astronomy at court. The tower stands near the royal tomb of King Naemul.
How it works
The tower has three parts: a base, a cylindrical body shaped like a bottle, and a square top. The body is made of 27 layers of fan-shaped stones, with a square opening in the 13th to 15th tiers facing slightly east of south. Below the opening the interior is filled with rubble; above it the tower is hollow. Protruding stone ends inside would have held a ladder. An old document says the structure was made for men to climb up by entering through the center, which implies observers went in through the opening and up to the top.
Who pushed back
The tower was first described as an observatory by the Japanese meteorologist Wada Yuji in a 1910 report, and Korean scholars presented it as evidence that Silla made its own astronomical observations. Dissenting views emerged in the 1960s. Yi Yong-beom argued that it was an altar modeled on Mount Sumi, and Park Seong-rae accepted it as an observatory only in a broad sense, seeing it as an altar to the god of agriculture.
What lasted
The numbers built into the tower have been read as calendar symbols. The body is counted as about 365 stones, for the days of the year; 27 tiers may mark Seondeok as the 27th ruler of Silla; adding the base gives 28, the number of East Asian constellations; and the 12 tiers above and below the opening recall the 12 months and 24 solar terms. The tower has kept its original appearance for about 1,300 years, though it now tilts slightly to the northeast.
Where it stands today
Cheomseongdae is designated National Treasure no. 31, owned by the Korean government and managed by the city of Gyeongju. It is open to visitors. Safety inspections have been carried out annually since 1981, an hourly measurement system was installed in 2007, and the structure was surveyed with a three-dimensional scanner in 2009.
