Jang Young-sil, the Greatest Scientist and Astronomer of the Joseon Dynasty
In 1438, the 20th year of King Sejong’s reign, Heumgyeonggak (흠경각), meaning “The Hall of Respectful Veneration,” was built in the Gyeongbok Palace. The Heumgyeonggak Pavilion housed innovative scientific devices of the Joseon Dynasty including the Ongnugi (옥루기), a jade water clock equipped with automatically moving dolls that indicated seasons and hours. The automatic water clock had a wheel that moved by the water, and 12 mechanical dolls would appear at every hour on the hour to tell the time. King Sejong was concerned with the ways of the heavens, and he wished to guide and inform his people about the times and seasons, so that people could better determine the best time to plant and harvest crops. The man who aided King Sejong to realize this goal was none other than Jang Young-sil, the greatest scientist and astronomer of the Joseon era.

A Man of Exceptional Scientific Ability Overcomes Class Barriers
Jang Young-sil’s exact date of birth remains unknown to this day. Historical records show that Jang’s mother was a gwangi (관기) or a gisaeng (기생), which is a Korean term that refers to female artists of inferior social status who entertained men including those from the ruling class or the king. Jang’s social status was a gwanno (관노), a servant in civil service district courts. Despite his inferior class status, Jang was known for his exceptional talent in inventing innovative scientific equipment. In 1400, when the Southeastern region of the peninsula was suffering from a persistent drought, Jang helped to relieve the problem by proposing the idea of storing water in a reservoir, which earned him a place in the royal court.

In an effort to lead the new renaissance of the Joseon Dynasty, King Sejong executed a policy that appointed individuals to various government posts solely based on one’s talent and skills rather than on his social status. In 1421, three years after King Sejong ascended the throne, Jang returned home after studying astronomy in earnest with scholars in China for one year. Although the Joseon society was characterized by distinct social classification at the time, Jang found favor in the eyes of King Sejong who allowed him to work for the government with commoner status. After making his entry into the royal palace, Jang displayed his outstanding abilities and invented numerous scientific devices that are believed to have fueled technological advances of the Joseon era.

In 1434, Jang invented Korea’s first automated water clock, called the Jagyeongnu (자격루). A sophisticated instrument that integrated mathematical, physical, and mechanical engineering principles in one, the Jagyeongnu is considered one of the greatest scientific developments in Korean history. The device worked the following way: three columns of water containing jars discharged 0.1 liter of water every one minute, allowing small iron beads to drop, which in turn let the automatically moving twelve wooden dolls erect, visually representing the time. When the beads ran into each other, a figurine struck a cymbal, announcing each hour. Jang was able to create a perfectly automatic instrument capable of maintaining a fixed water pool while discharging water at regular time intervals to collide with the time indicators. In addition, Jang created Korea’s first sundial called the Angbu Ilgu (앙부일구) and the world’s first rainwater gauge called Cheokugi (측우기) among many other notable inventions.

According to historical records, in 1442, King Sejong’s gama, or Korean sedan chair, broke while Sejong was on his trip to a hot spring in Icheon. Jang had built the chair and was held accountable for the accident, and was soon dismissed from the royal palace. Although much about the latter days of Jang’s life including his death is shrouded in mystery and speculation, Koreans will forever remember Jang for his great achievements that helped Joseon to lead the technology race.