Born the property of the state, he built the clock that told a kingdom the hour — then the record simply stops.
In the palace of Gyeongbokgung, in 1434, a jar of water began to tell the time by itself. Water seeped from vessel to vessel; a float rose; a rod tipped a board of small iron balls; one ball rolled and struck a larger one, and that ball swung a hammer at a gong. A wooden figure lifted a placard naming the hour. No servant watched the water. No drummer counted. Joseon had a clock that struck its own hours — and the man who built it had been born the property of the state.
His name was Jang Yeong-sil. His mother was a government-registered courtesan, which under Joseon law made him nobi, of the lowest class; his father's family had come from Yuan China. By every rule of his world he should have spent his life at a provincial forge. Instead King Sejong brought him into the court, and for two decades he built the instruments by which a country measured the sky, the rain, and the passing hour. Then, in 1442, a royal sedan chair broke — and the annals that had praised him fall silent for good.
From nobi to the Hall of Worthies
The lifeline
c. 1390
Born in Gyeongsang province to a courtesan mother — by law a government-owned slave
1422–23
Sejong appoints him to office over ministers who object to “the child of a kisaeng”
1433
Builds the water-driven armillary sphere; the court grants a slave-born man the rank of hogun
1434
Completes the self-striking water clock; bronze sundials go up in the public streets
1438
The Ongnu — an automated jade clepsydra built around an artificial mountain
1442
The king's sedan chair breaks. Eighty lashes, expulsion — and no further record of him
By the numbers
The measure of him
80
Lashes ordered in 1442, after the royal sedan chair he had overseen broke on the road
12
Zodiac animals engraved on his public sundials, so that people who could not read could still read the hour
198
Years between Joseon's state rain gauge of 1441 and Europe's first, Castelli's at Lake Trasimeno in 1639
0
Recorded date of his death. After 1442 the annals never mention him again
What he built for a kingdom
The instruments
1433
The honcheonui. An armillary sphere turned by a waterwheel, so that the positions of sun, moon and stars could be read by day or by night.
1434
The Jagyeokru — the self-striking water clock. Floats, tilting boards, rolling iron balls, levers and puppets, sounding bell, drum and gong. It became Joseon's national standard of time.
1434
The angbu-ilgu. Hemispherical bronze sundials, seven curved hour-lines for the seasons, set at the Hyejeong bridge and outside Jongmyo shrine — public clocks, in a world where time belonged to kings.
1434
Gabinja type. With the scholars of the Hall of Worthies he helped cast a copper-zinc and lead-tin font said to print twice as fast as its predecessor — and recast six times over the next 370 years.
1438
The Ongnu, a jade clepsydra staged on a paper mountain: immortals, warriors and twelve gods that moved with the hours and tracked the sun's course through the year.
1441
The supyo water gauge — a calibrated stone column in the stream — and the cheugugi, a standard rain gauge issued to provincial offices: the world's first state rainfall network. (The annals credit the crown prince with proposing the rain gauge; Jang's role is traditional, not documented.)
Praised, then erased
Recognition
In his lifetime
Born nobi — the lowest class in Joseon, owned by the state
Ministers argued a courtesan's son could not hold office at all
Flogged eighty times in 1442 and expelled from the palace
No portrait, no death date; the record ends mid-life
His instruments decayed — no sundial of Sejong's reign survived the invasions of 1592–98
Today
Surviving parts of the water clock are National Treasure No. 229 of South Korea, designated 3 March 1985
Held by the National Palace Museum of Korea in Seoul
A full working reconstruction of the striking clock was completed in 2007
Korea's leading industrial technology prize has carried his name since 1991
Google marked him with a Doodle on 19 May 2018; he is a fixture of Korean schoolrooms and screens
The place that held his work
Gyeongbokgung
His instruments stood in the palace at Hanseong — sundials and clocks about the Gyeonghoeru pavilion, the striking clepsydra in the Borugak, the jade clock in its own hall, the Heumgyeonggak. From that one courtyard the hour was carried out to the whole kingdom. Then the palace burned, and most of what he made was lost.
1395
The year the palace was founded, four decades before his clock was installed in it
1592
Invasion and fire; no Sejong-era sundial is known to have survived
93.5 cm
Across — the bronze water cylinders of the clock, still held in Seoul
"Though he followed my instruction, without this man it could never have been made. He has fashioned an instrument to be handed down ten thousand generations; his merit is not small."
— King Sejong, granting him the rank of hogun Veritable Records of the Joseon Dynasty, 1433