Gyupyo (圭表, Gnomon) consists of Gyu (圭, Measuring Scale) and Pyo (表, Column), and was one of the traditional astronomical instruments in East Asia. Daegyupyo (Large Gnomon) was manufactured in the Joseon dynasty around 1434 ~ 1435. To increase the measurement accuracy, it was equipped with a Hoengyang (橫梁, Cross-bar) and used a Youngbu (影符, Shadow-Definer) which was invented during the Yuan dynasty (1271 ~ 1368). The cross-bar was installed on the top of the column and this structure was called Eol (臬). In addition, three plumbs hanging from the cross-bar was employed to vertically built Eol on the measuring scale. This method was also used to not only check the vertical of Eol but also diagnose the horizontal of the cross-bar. Throughout this study, we found that a cross-bar in a gnomon has played three important roles; measurement of the shadow length made by the central part of the Sun, increase of the measurement precision using the shadow-definer, and diagnosis of the vertical of Eol and the horizontal of the cross-bar itself using the three plumbs. Hence, it can be evaluated that the employment of a cross-bar and a shadow-definer in a gnomon was a high technology in the contemporary times. In conclusion, we think that this study is helpful for understanding the Large Gnomon of the Joseon dynasty.

In this study, the characteristics of a horizontal sundial from the Joseon Dynasty were investigated. Korea’s Treasure No. 840 (T840) is a Western-style horizontal sundial where hour-lines and solar-term-lines are engraved. The inscription of this sundial indicates that the latitude (altitude of the north celestial pole) is 37° 39´, but the gnomon is lost. In the present study, the latitude of the sundial and the length of the gnomon were estimated based only on the hour-lines and solar-termlines of the horizontal sundial. When statistically calculated from the convergent point obtained by extending the hourlines, the latitude of this sundial was 37° 15´ ± 26´, which showed a 24´ difference from the record of the inscription. When it was also assumed that a convergent point is changeable, the estimation of the sundial’s latitude was found to be sensitive to the variation of this point. This study found that T840 used a vertical gnomon, that is, perpendicular to the horizontal plane, rather than an inclined triangular gnomon, and a horn-shaped mark like a vertical gnomon is cut on its surface. The length of the gnomon engraved on the artifact was 43.1 mm, and in the present study was statistically calculated as 43.7 ± 0.7 mm. In addition, the position of the gnomon according to the original inscription and our calculation showed an error of 0.3 mm.