Jang Yeong-sil

Jang Yeong-sil was a scientist and inventor active during the reign of King Sejong of Joseon. Though born into the lowest social class, his exceptional skill won the king's favor and earned him an official post at the royal supply office. He built the Jagyeongnu (a self-striking water clock), the Angbu-ilgu sundial, the Honcheonui armillary sphere for observing the heavens, and the Cheugugi rain gauge, contributing greatly to the advancement of astronomy and science in Joseon.