We study the manufacturers of the astronomical instruments invented as part of the so-called Ganui-Dae (astronomical platform) project in the reign of King Sejong (1418-1450) of the Joseon dynasty. The main purpose of this project was to compile the Chiljeongsan-Naepyeon, a Korean calendar, that lasted for 5.5 years from 1432 to 1438. Through this project, a total of 17 astronomical instruments such as the armillary sphere, celestial globe, and auto-striking clepsydra were developed. It is generally known that most of them were manufactured by Lee Cheon (李蕆, 1376-1451) and Jang Yeong-Sil (蔣英實, fl. 1423-1442). In this study, we investigate the accounts of the Veritable Records of King Sejong, Munjong, and Sejo, focusing on the inscription written out to memorize the completion of the Ganui-Dae project. We found that at least 12 persons took part in manufacturing the astronomical instruments of the Ganui-Dae project. Lee Cheon was involved in the production of four instruments, while Jang Yeong-Sil was involved in two; therefore, it seems that the achievement of Jang Yeong-Sil was overestimated. We also found that Jeong Cho (鄭招, ?-1434) was a scientist and Lee Cheon was an engineer in the early phase of the Ganui-Dae project, while King Sejong and Yi Sun-Ji (李純之, 1406-1465) played major role in the late phase. According to our study, King Sejong and Yi Sun-Ji were involved in the production of at least seven and five instruments, respectively. In conclusion, we believe that this study will be helpful to understand the practical manufacturers of the astronomical instruments invented during the Ganui-Dae project in the Joseon dynasty.

We report the results of our study for ascertaining whether Ganui-dae built in the Josen Dynasty actually performed an astronomical role or not. The Ganui-dae was the first astronomical structure built as a part of the state astronomical undertakings by King Se-Jong. Our analysis was based on the Annals of the Joseon Dynasty. At first we extracted the records regarding Ganui-dae from the Annals of the Joseon Dynasty and then classified them under six heads according to their astronomical meaning and historical significance. From this analysis we found that the Ganui-dae performed the actual astronomical role. In addition, the provisional offices and peoples mentioned in the records show the astronomical correlation. Generally, when taking into account the functional side of the records, the Ganui-dae was related with the observation. Therefore, the Ganui-dae was the space for the astronomical activity. In conclusion, the Ganui-dae was built for the purpose of the astronomical activity.

Since the thirteenth century, large scale facilities and various instruments for astronomical observation were built and installed in East Asia. During the Yuan Dynasty, S. ti.ntai (Beijing astronomical observatory in the Yuan Dynasty, 司天臺) was built in Beijing in 1279. Various astronomical instruments, including Ganui (Jianyi, simplified armillary sphere, 簡儀), Yang-yi (upward hemisphere, 仰儀) and Gyupyo (gnomon, 圭表) were installed in this observatory. These astronomical instruments were modified and improved by researchers of the Joseon Dynasty. Ganuidae (Joseon astronomical observatory, 簡儀臺) was built in Gyeongbokgung (or Gyeongbok palace, 景福宮), Seoul. Its scale was 31 Cheok (Korean feet in the Joseon Dynasty, 尺) in height, 47 Cheok in length and 32 Cheok in width. Lee, Cheon (李蕆, 1376~1451), a responsible leader of Ganuidae project, set up various astronomical instruments with his colleagues. Ganui and Jeongbangan (direction-determining board, 正方案) were installed at the top of this observatory. Gyupyo was installed at the west side of this observatory and Honui (armillary sphere, 渾儀) and Honsang (celestial globe, 渾象) were installed in a small pavilion which was located next to Gyupyo. A decade after installation, this observatory was moved to the north-west side of the palace but almost destroyed during Japanese invasion of Korea in 1592 except Ganuidae. We have analyzed documents about Ganuidae and investigated Chinese remains of astronomical observatories and artifacts of astronomical instruments. In this paper, we suggest the appearance, structure, arrangement and scale of Ganuidae, which are expected to be used for the restoration of Ganuidae at some day in the near future.