[Culture & Heritage] Reading Time Through Shadows: The Science of Joseon'...

Portable Angbuilgu sundial. Photo by National Museum of Korea.

In modern society, the system of 24 hours in a day and 365 days in a year functions as a basic unit of time. Before standard time and mechanical clocks became widespread, the sun itself served as the primary reference for measuring time. Because the position of the sun alone could not provide precise readings, sundials emerged as instruments that interpreted the length and direction of shadows cast by sunlight.

A sundial uses sunlight and a fixed rod that creates a shadow. As long as the Earth rotates, the instrument functions continuously. Its mechanism is relatively simple and resistant to mechanical failure. As technology developed, smaller portable versions were also produced. Before mechanical clocks became common, sundials were widely used across many civilizations.

The representative sundial of Joseon was the Angbuilgu (仰釜日晷), created during the reign of King Sejong the Great. The word “Angbu” refers to the concave shape of a cauldron facing upward, while “Ilgu” means the shadow of the sun, referring to a sundial. Unlike ordinary flat-surfaced sundials, the Angbuilgu used a hollow hemispherical structure.

◇ A Sundial Installed Along Public Streets

Records in the “Annals of the Joseon Dynasty” state that Angbuilgu sundials were installed in 1434, the 16th year of King Sejong’s reign, at Hyejeonggyo Bridge and in front of Jongmyo Shrine. At the time, the main east-west avenue of Hanyang passed through those locations and connected Seodaemun and Dongdaemun. In effect, public clocks were installed along one of the busiest roads in the capital.

The purpose of the Angbuilgu also appears in an inscription written by Kim Don (1385–1440). He explained that degree markings and illustrations of the 12 zodiac animals were engraved inside the hemispherical surface so that even people unable to read written characters could estimate the time using the animal symbols.

The installation of the sundials along public roads reflected the same intention. While astronomical instruments inside the palace primarily served the royal court and government offices, the Angbuilgu functioned as a public device that informed ordinary people of the time. It demonstrated how scientific technology during King Sejong’s reign was applied to practical daily use.

◇ From the Yuan Dynasty’s Angui to Joseon’s Angbuilgu

Texts compiled during the reigns of King Sejong and King Jeongjo mention “Angui,” an astronomical instrument associated with Guo Shoujing of the Yuan Dynasty, as a model for the Angbuilgu. Historical records describe the Angui as a large hemispherical astronomical instrument capable not only of functioning as a sundial but also of observing solar and lunar eclipses.

No surviving artifact or confirmed record proves that the Angui was actually produced and used. If it existed, it was likely installed within palaces or astronomical institutions as a symbol of imperial authority. In East Asian dynasties, observing celestial movements and regulating calendars were considered responsibilities of the ruler.

Astronomical observation and calendar reform also formed an important part of governance in Joseon. Calculating agricultural seasons and tracking celestial movement were directly connected to state administration. During King Sejong’s reign, several scientific instruments were produced alongside the Angbuilgu, including the Ilseong Jeongsi-ui, which measured time through the sun and stars, and the Jagyeokru water clock equipped with automatic striking mechanisms.

King Sejong did not treat astronomical instruments merely as symbols of royal authority. By installing Angbuilgu sundials along public streets and including zodiac illustrations, he made timekeeping accessible to ordinary citizens. The design and placement of the instrument reflected the people-centered philosophy associated with his rule.

Bronze Angbuilgu sundial. Photo by National Museum of Korea.

◇ Measuring Daily Hours and the Seasons of the Year

No Angbuilgu from King Sejong’s period survives today. Existing examples were mostly produced between the 17th century and the late 19th century during the later Joseon period. Although their materials and sizes vary, the overall structure remains largely consistent. A bronze Angbuilgu housed at the National Palace Museum of Korea and designated as Treasure No. 845 is believed to be closest in form to the original Sejong-era model.

Inside the hemispherical surface, engraved lines divide the instrument into measurement sections. Around the upper rim, flat outer surfaces contain directional markings and inscriptions. The sundial displays 24 directions using combinations of the 12 zodiac signs, the eight stems, and the four trigrams at 15-degree intervals.

The gnomon, or shadow-casting pointer, was placed in the true south position. Time lines were engraved as spherical longitudinal lines running north to south, while latitudinal lines functioned as seasonal markers. The names of the 24 seasonal divisions were written at the ends of the seasonal lines.

The seasonal line touched by the shadow indicated the corresponding seasonal period, while the intersecting time line showed the hour of the day. In this way, the Angbuilgu measured both daily time and the annual seasonal cycle simultaneously.

◇ From the 100-Gak System to the 96-Gak System

Differences between Sejong-era Angbuilgu sundials and later Joseon examples can be found in their systems of time division. Traditional Korean timekeeping used a 12-hour, 100-gak system. A day was divided into 12 periods named after the zodiac signs, with each period further divided into “Cho” and “Jeong,” producing 24 subdivisions in total.

At the same time, another system divided the day into 100 gak units. Since the 100-gak system was used during King Sejong’s reign, researchers believe the original Angbuilgu likely included those markings.

In 1653, Joseon adopted the Shixian calendar system and changed to a 12-hour, 96-gak system. All surviving Angbuilgu examples follow the later 96-gak structure, which is one reason they are believed to have been produced after 1653. The 96-gak system can also be converted relatively accurately into modern time units.

◇ Portable Sundials That Fit in the Palm of a Hand

Clockmaking traditions continued into the later Joseon period. Astronomical clocks combining armillary spheres and striking mechanisms were produced, while the Angbuilgu evolved into portable forms. In addition to fixed sundials installed in government offices, homes, and palace courtyards, small handheld versions also appeared.

Among the best-known examples are portable Angbuilgu sundials produced by brothers Kang Yun (1830–1898) and Kang Geon (1843–1909). These works are particularly valuable because both the makers and production dates are clearly recorded.

A portable Angbuilgu housed at the National Museum of Korea and designated as Treasure No. 852 was created by Kang Geon. Measuring roughly 5.6 centimeters tall, 3.4 centimeters wide, and 2 centimeters high, it is small enough to fit in the palm of a hand. The frame was made from gray-white marble, while the interior contains a concave hemispherical dial with engraved lines and a bronze gnomon.

A cylindrical section beside the dial contains a compass. Because portable sundials required accurate directional alignment, the compass helped users orient the instrument correctly.

Front and back of the portable Angbuilgu sundial. Photo by National Museum of Korea.

The front bears the inscription “Angbuilgu.” Inside the dial are the 12 zodiac symbols, time lines marked at 30-minute intervals, and 13 seasonal division lines. The names of the 24 seasonal divisions are engraved along the sides, while the compass displays the 24 directions.

An inscription on the underside records that the sundial was made by Kang Geon of Jinju in late spring of 1871.

The National Palace Museum of Korea also houses portable Angbuilgu works by both Kang Yun and Kang Geon. Materials and forms vary by piece, and one example made by Kang Yun uses ivory.

The two brothers were great-grandsons of scholar and painter Kang Sehwang (1713–1791). Both are regarded as important clockmakers of the late Joseon period. Kang Yun reportedly presented a portable sundial to King Gojong and later participated in the production and restoration of royal seals. Their father, Kang I-o, was also known for producing astronomical clocks, making the family an example of scientific craftsmanship passed down across generations.

The Angbuilgu was a clock based on sunlight and shadows. During King Sejong’s reign, it functioned as a public clock installed along city streets. In the later Joseon period, its structure and measurement systems evolved, and portable versions also appeared. Within its small hemispherical form, the Angbuilgu combined daily timekeeping, seasonal calculation, directional measurement, and the technical skill of Joseon-era science.

Reported by News Culture M.J._mj94070777@nc.press

* This article was manually prepared as a translated piece and later proofread and reviewed with the assistance of AI.