The Silla kingdom was one of three that existed on the Korean peninsula from 57 B.C. In 660 and 668 AD. managed to conquer the territories of the other two, dominating the entire region for almost three centuries, until 935 AD. During all that time the capital of the kingdom was the city of Gyeongju.
Today the town is the capital of North Gyeongsang Province and is home to a large number of Silla period temples, mausoleums and monuments, so many that the area forms a national park that has been declared a World Heritage Site, and is often referred to as the museum without walls .
Among the monuments found there are the Buddhist grotto of Seokguram, the Bulguksa temple, the historical areas of Kyongju with the Wolseong Palace, the Tumuli Park, the Sanseong Fortress, and the Yangdong traditional village, among others.
And also the Cheomseongdae, the oldest astronomical observatory in all of Asia that is still standing, built in 647 AD. during the reign of Queen Seondeok, who died that same year and was Silla's first queen, and the second in East Asian history.
Since seasonal changes were very important in making agricultural decisions, astronomy was especially important at the time it was built. Furthermore, given the importance of astrology at the time, astronomical observation was closely linked to politics. For this reason, in the Silla kingdom there was a great interest in astronomical observations, and it is in this context that Cheomseongdae was built.
His name, which in Korean means stargazing tower , perfectly describes its architecture, since it is a 9.17-meter-high tower in the shape of a bottle.
Its base is 5.7 meters wide and is built with a single layer of 12 rectangular stones (which could symbolize the 12 months of the year). The tower is filled with earth and rubble up to the level of the only window, located halfway up the cylindrical column that forms its body and facing south (slightly east of south). Its shape is inspired by an earlier observatory, Baekje's Jeomseongdae, which is known from historical records.
The cylindrical body of the tower is built with 365 pieces of carved granite that symbolize the number of days in the year. An additional stone slab, located inside the top of the tower, is not visible from the outside. Each of the 365 stones is in the shape of a curved or folded rectangle, to fit the cylindrical structure of the whole.
It has 27 levels or rows of stones, which may reflect the fact that Queen Seondeok was the 27th monarch of the Silla kingdom. If the base is included, the number would be 28, which corresponds to the 28 constellations of East Asia.
The window separates the central column into 12 layers of stones, both above and below, symbolizing the 12 months of the year and the 24 solar terms (the climatic terms that represent the changes of the seasons according to the Chinese calendar, and according to the position of the Earth in the orbit around the sun).
At the top of the tower there was a device or artifact for observing the sky, although its nature is unknown. However, the structure that allowed its installation remains, two interlocking stones from which four corners protrude.
On levels 19-20 and 25-26 there are ends that stick out, in order to be able to place a ladder to enter and exit the building (through the window or the top).
The Cheomseongdae influenced the construction of the Japanese Senseidai observatory in 675 AD. and from Duke Zhou's observatory in China, in 723 AD
The first to realize that the tower had been an astronomical observatory was the Japanese meteorologist Wada Yuji, who in 1910 published his theory that an observation balloon or an astronomical clock could be installed on top of a structure of wood.
The original appearance and form of the Cheomseongdae have remained unchanged for more than 1,300 years. In recent times, the structure leaned slightly to the northeast, which made it necessary to install a continuous monitoring system that monitors structural displacements and cracks.