Sejong's first assignment to Jang was to build a celestial globe to measure astronomical objects.
Books obtained from Arabian and Chinese scholars were not complete in their instructions, for these devices could also be used for military purposes.
After two months of study, Jang made a spherical device that could perform with mediocre accuracy.
In 1433, a year after his first attempt, Jang made an armillary sphere known as the honcheonui (혼천의, 渾天儀).
Honcheonui depended on a waterwheel to rotate the internal globe to indicate time.
Whether day or night, this allowed the instrument to be updated on the positions of the sun, moon, and the stars.
Later celestial globes (i.e. gyupyo (규표)) could measure time changes according to the seasonal variations.
These instruments, along with the sundials and water clocks, were stationed around the Gyeongnghoeru Pavilion in Gyeongbok Palace and made into use by the astronomers.
The success of Jang's astronomical machines was marked in 1442 AD when the Korean astronomers compiled their computations on the courses of the seven heavenly objects (five visible planets, the sun, and moon) in Chiljeongsan (칠정산), an astronomical calendar that made it possible for scientists to calculate and accurately predict all the major heavenly phenomena, such as solar eclipses and other stellar movements.
Sejong's first assignment to Jang was to build a celestial globe to measure astronomical objects. Books obtained from Arabian and Chinese scholars were not complete in their instructions, for these devices could also be used for military purposes. After two months of study, Jang made a spherical device that could perform with mediocre accuracy. In 1433, a year after his first attempt, Jang made an armillary sphere known as the honcheonui (혼천의, 渾天儀). Honcheonui depended on a waterwheel to rotate the internal globe to indicate time. Whether day or night, this allowed the instrument to be updated on the positions of the sun, moon, and the stars. Later celestial globes (i.e. gyupyo (규표)) could measure time changes according to the seasonal variations. These instruments, along with the sundials and water clocks, were stationed around the Gyeongnghoeru Pavilion in Gyeongbok Palace and made into use by the astronomers. The success of Jang's astronomical machines was marked in 1442 AD when the Korean astronomers compiled their computations on the courses of the seven heavenly objects (five visible planets, the sun, and moon) in Chiljeongsan (칠정산), an astronomical calendar that made it possible for scientists to calculate and accurately predict all the major heavenly phenomena, such as solar eclipses and other stellar movements.
Jang Yeong-sil