This study examines an 8-ja (1,66 m) gnomon built by the Korea Astronomy and Space Science Institute (KASI, 127°22′32″ N, 36°23′57″ E) in 2011. This gnomon is an astronomical instrument with the same function as the Small Gnomon, which was built in 1440 during the reign of King Sejong of the Joseon Dynasty. The length of the column’s (or crossbar’s) shadow cast by the sun at the meridian passage was measured for a total of 303 days out of the 1,492-day observation period, which lasted from December 2015 to December 2019. The shadow lengths showed a measurement error of -0.8 to 1.2 cm compared to modern calculations. Furthermore, this study also estimated the time of the winter solstice using shadow lengths obtained from modern calculations for 50 days before and after the winter solstice. This calculation method was first introduced in the Daming Calendar (462) by Zu Zhongzhi (祖冲之, 429-500) and was applied to the Shushi Calendar (1281) by Guo Shoujing (郭守敬, 1231~1316). The time of the winter solstice did not demonstrate a constant value on the days before and after the winter solstice but showed a decreasing pattern, which had a constant slope each year. The tropical year can be obtained from the time of the winter solstice of two consecutive years. The fractional part of the tropical year (0.242 189 days) was estimated 0.242 789 ± 0.003 570 days in 2015-2016 (using data from 23 days both before and after the winter solstice) and 0.242 480 ± 0.004 616 days in 2016-2017 (using data from 45 days both before and after the winter solstice). Ultimately, the length of the tropical year estimated from the shadow lengths measured by the KASI’s 8-ja gnomon achieved an accuracy of 365.24 days. The observation value of the 8-ja gnomon showed an error of 0.1624 (±0.1229) days. It was found that this actual measurement error could result in an error of 3.9 h in the estimation of the time of the winter solstice or the accuracy of the length of the tropical year.

During the reign of King Sejong in the Joseon Dynasty (1433-1438), the Daegyupyo (large gnomon) was produced. The Daegyupyo, with a crossbar (horizontal bar), was used to observe the length of the gnomon’s shadow cast by the sun passing at the meridian. The shadow of this crossbar can be obtained using a measurable device called the Yeongbu (shadow definer). These Daegyupyo and Yeongbu are described in detail in the “Treatise on Astronomy” of Yuan History or “Celestial Spheres and Globes” of Jega-Yeoksang-Jjp (Collected Discourses on the Astronomy and Calendrical Science of the Chinese Masters). According to Jega-Yeoksang-Jjp, the Yeongbu had a structure similar to a door attached to its frame. A pinhole is located in the center of a copper leaf corresponding to the door of the Yeongbu. The image of the sun’s meridian transit and the shadow of the crossbar through the pinhole are projected onto the surface of the Daegyupyo’s ruler stone. Unlike the width and length of the Yeongbu, the height of the Yeongbu is not recorded. This research analyzed the height of the Yeongbu required to maintain the constant distance from the pinhole to the ruler stone surface. Based on these assumptions, it was estimated that 8 to 13 Yeongbu of different heights would be needed for observations using the Daegyupyo in Seoul. To accommodate the need for Yeongbu of various heights, this study proposed a model for a stackable Yeongbu with an adjustable height.

Numerous Sundials were fabricated during the reign of King Sejong of the Joseon Dynasty. One among them is Jeongnam-Ilgu (the Fixing-South Sundial), where the time can be measured after setting up the suitable meridian line without a compass. We reconstructed the new Jeongnam-Ilgu model based on the records of ‘Description of Making the Royal Observatory Ganui (簡儀臺記)’ in the Veritable Record of King Sejong. Jeongnam-Ilgu has a summer solstice half-ring under a horizontal ring which is fixed to two pillars in the north and south, and in which a declination ring rotates around the polar axis. In our model, the polar axis matches the altitude of Hanyang (that is Seoul). There are two merits if the model is designed to install the polar axis in the way that enters both the north and south poles and rotates in them: One is that it is possible to fix the polar axis to the declination ring together with the cross-strut. The other is that a twig for hanging weights can be protruded on the North Pole. The declination ring is supposed to be 178 mm in diameter and is carved on the scale of the celestial-circumference degrees on the ring’s surface, where a degree scale can be divided into four equal parts through the diagonal lines. In addition, the time’s graduation that is drawn on the summer solstice half-ring makes it possible to measure the daytime throughout the year. An observational property of Jeongnam-Ilgu is that a solar image can be obtained using a pin-hole. The position cast by the solar image between hour circles makes a time measurement. We hope our study will contribute to the restoration of Jeongnam-Ilgu.

This study investigated the stone Angbu-ilgu (scaphe sundial) of the Korea Meteorological Administration (KMA) and the Seoul Museum of History (SMH). Since the first Angbu-ilgu was produced in Korea in 1434 (the year of the reign of King Sejong), Angbu-ilgu has been reproduced with various materials. The upper surface of these two stone Angbu-ilgus symbolizes the horizon. On the hemisphere concave at the center of the horizon, the South Pole, the time line, and the season line are engraved. On the horizon of both the KMA and SMH Angbu-ilgus, the schematic, typeface, and composition of the inscription completely coincide with each other. In this study, it was estimated that the appearance of the KMA Angbu-ilgu, which was damaged at some point previously at least once, was similar to that of the SMH Angbu-ilgu, and this means that it is superficially similar with Treasure No 840, the stone horizontal sundial. In the concave hemisphere of both the stone Angbu-ilgus of the KMA and SMH, there are hour lines and 24 solar-term lines (13 line), and there is an intersection point where these lines meet the horizon, respectively. It can be verified that these intersections of these two Angbu-ilgus can be calculated as having a latitude of +37°39′15″. The hour lines of the two stone Angbu-ilgus show that they were made after about 1900.

We report a calendar sheet for the 31st year of the reign of King Gojong (1894) (hereafter, calendar sheet 1894) in Korea, which calendrical data in a single page. This calendar sheet 1894 is composed of 14 rows by 14 columns (about 190 cells), and various calendrical data are recorded such as the sexagenary circle of the first day in each month, 24 solar terms, full moon day. In this paper, we compare calendrical data of 1894 calendar sheet with those of the almanac based on the Shixian calendar (hereafter, annual almanac) of the same year. Our findings are as follows. First, we find that the year is expressed using the reign-year of the king of the Joseon dynasty differently from using the reign-style of China in the annual almanac those times. Other calendar days of this calendar sheet are the same as those of the annual almanac in term of lunar dates, 24 solar terms, sexagenary days and so forth. Second, we find that the calendar sheet 1894 contains memorial days for 64 lineally ancestors of the Joseon royal family. These royal memorial days appears in the annual almanac two years later (i.e., 1896). Third, as the most distinctive feature, we find that the symbol of 工 kept every two cells. It was found that the cells can be filled with three days as the maximum number of days and then are labelled the same symbol 工 every second cell. This feature allows us to get the first year in which this kind of calendar sheet was published. It is conjectured one of 11 years, such as 1845, 1846, 1847, 1873, 1874, 1875, 1876, 1877, 1878, 1879 or 1880. We also think that the format of the calendar sheet 1894 has influenced on the Daehan-Minryeok (Korean civil calendar sheet) of 1920.