In this paper, we investigate the sundials made by the Kang Family of Jinju in the later period of the Joseon dynasty in terms of their characteristics, functions and the manufacturing technique. One of the characteristics of these sundials is that the value of polar height (i.e., the latitude of Seoul), the name of manufacturer, 24 seasonal subdivisions and so forth are written on the surface. In particular, polar height is expressed as ‘37˚ 39′ 15′′’ in all 12 sundials examined in this study. To investigate the manufacturing technology in terms of astronomy, we analyze the positions of gnomon and of the lines corresponding to 24 seasonal subdivisions (season lines) and to each hour (hour lines). To verify the accuracy of the positions, we use a planar projection method. That is, we obtain 2D images of the sundials using a camera or scanner, and compare these with astronomical calculations for the positions of gnomon and season/hour lines. We believe that this method will be very useful for the study of sundials preserved elsewhere.
It is well known that
In this paper, we investigate physical characteristics and detailed functions of 12 sundials with an express statement so identify as the person of the
We also analyze the positions of
In previous research on the sundials of the Joseon dynasty period, Jeon (1963) studied their structure and characteristics, making reference to various literatures and discussing these in terms of history. Later, Lee (1982) performed research on the sundials of the Joseon dynasty, focusing on their history and scientific principles. In the process of studying the astronomical instruments that dated from the reign of King
As mentioned above, thus far all studies on sundials have maintained a focus on history and scientific principles at the base of the literature, or on newly restored sundials of the early Joseon dynasty. In terms of portable sundials, which were a representative sundial type of the later Joseon dynasty, only their introduction and express statement were dealt with briefly in several studies (e.g., Jeon 1974, Jeon 1994a, b, 2000, Korea Foundation for the Science and Culture 1997, National Folk Museum 2004, Needham & Lu 1986, Seoul Museum of History 2004). It was Jeon (1994a) who first studied
1an ancestral seat 2‘-gong’ means a title for a high officer.
2. SUNDIALS OF THE KANG FAMILY OF JINJU
In Fig. 2, we summarize the table of descent from the
Of five sons of
According to
2.2 Sundials of the Kang Family
In Table 1, we summarize the sundials that can be clearly identified as being built by the
Sundials made by the
In the hemispherical dial plate, 13 season lines are marked: two for
[Table 1.] Sundials made by the Kang Family.
Sundials made by the Kang Family.
For inventory no. 2, a metal rod (
In Fig. 5, we present a scaphe sundial of Japan and a western portable sundial. As can be seen in Fig. 5a, the Japanese scaphe sundial consists only of a main body and the cover made of ivory, but its structure as a whole is similar to that of a Korean scaphe sundial: a hemispherical dial plate and a compass. Regarding the compass part, the north and south directions are clearly written in Chinese characters, and a twisted gold cord is present around the rim, serving a decorative function. At the center of the dial plate, there is a hole for the insertion of
Fig. 5b shows a western scaphe sundial made around the 18th century (Rohr 1996). The shape of the dial plate is hemispherical but less round than a Korean scaphe sundial, and
For the sundials made by the
The portable sundial manufacturing technology of the later Joseon dynasty originated from the 15th century tradition of making portable equatorial sundials, such as
3During the Joseon dynasty period, Yeong-uijeong concurrently held the directorship of Gwansang-gam. 4According to the Sebo, Yun Kang was born in May of Kyungsin year (1810), Ho Kang (his older brother) in October of Gimi year (1819) and Hong Kang (his younger brother) in Musul year (1838). Considering the relation with his brothers and the history of his official ranks, we think that Yun Kang was actually born in Kyungin. That is, that Kyungsin is a typo of Kyungin. 5In the Joseon dynasty, a man could obtain a government position without undergoing the civil service examination if his father/ancestor was a meritorious subject or a high official.20 6Office that took charge of construction and civil engineering works in the later Joseon dynasty. 7Refer to http://sillok.history.go.kr 8Although it is known that two other sundials, one preserved in a clock company (http://www.timeseoul.com) and the other exhibited to a TV program (KBS Authentic and Master Goods; 7 August 2003), were also made by Yun Kang, we could not include these in this study.
3. PLANAR PROJECTION OF SEASON AND HOUR LINES
3.1 Construction of season and hour lines
To draw season lines on a scaphe sundial, we have to know the real altitude of the sun in each season using the declination of the sun at that moment and the latitude of a site (Nha 1995). Fig. 7 shows a conceptual diagram of the device drawing a season line on a scaphe sundial: first, produce a polar axis rotational semicircle that can rotate around the axis of NPCS (at this time, the position of SPCS becomes that of
3.2 Planar projection of season and hour lines
In general, the investigation of a scaphe sundial is to directly measure the physical size and the intervals of the season and hour lines. Sometimes, a three-dimensional (3D) scanning method is used to accurately measure the intervals drawn on the inside. However, this method is limited in terms of its practical application due to its high price. So if the purpose of the investigation is simply measurement rather than production of a replica, 3D scanning is considered impractical. In this study, we develop a method that can be used to investigate the accuracy of season and hour lines using a 2D image.
There are two methods that can be used to obtain a 2D image: taking a photograph, or using a scanner. In particular, the latter method is very useful for objects with a flat face that can be closely adhered to a plane, such as a scaphe sundial. With a plane sundial, it is possible to scan the face of the sundial after separating or folding
3.2.1 Planar projection of season lines
Twenty-four seasonal subdivisions are the quantities defined in the ecliptic coordinate system. That is,
(i) Declination (δ)
where is the obliquity of the ecliptic and varies according to the period. As the sundials used in this study were manufactured between 1860 and 1908 (see Fig. 6), the maximum and minimum values of are 23.449 (in c. 1885) and 23.460 (in c. 1875) degrees, respectively (Meeus 1998). Nonetheless, we use a constant value of 23.5 degrees based on
(ii) Azimuth (
where φ is the observer’s latitude, and
(iii) Projection into the
where
When projecting the season line of
SCoordinates1 of projected season lines with respect to the observer’s latitude and the obliquity of the ecliptic.
3.2.2 Planar projection of hour lines
A sundial is a clock that measures time according to the apparent motion of the sun, and hence its scale marks, or hour lines, indicate the apparent solar time. That is, it is noon when the sun is positioned at the local meridian (i.e.,
3.2.3 Planar projection according to the site
In Fig. 8, we present season (b-d direction) and hour (a-c direction) lines projected into a plane with respect to the observer’s latitude. In the panel of the legend, the length of a-c or b-d represents the diameter of a scaphe sundial, while o represents the location of the tip of
When we assume the length of a - c as unity, then that of a - o is 0.5. The lengths of a - s and a - w are the distances to the
According to astronomical calculations, the lengths of a-sp, a-s and a-w are 10.4 mm, 62.2 mm and 93.8 mm, respectively, when φ is 37˚ 34′ and the diameter of the dial plate is 100 mm. On the other hand, these are 31.2 mm, 186.6 mm and 281.4 mm when the latitude is the same but the diameter is 300 mm. In Table 4, we present the lengths of a-sp, a-s and a-w with respect to the observer’s latitude, assuming the diameter of 30 mm (i.e. traditional size of a portable scaphe sundial). As can be seen in the table, the differences caused by using a different latitude are very small. In this case, it is hard to discern the effect of the observer’s latitude using season lines. On the other hand, it is possible to distinguish the effect on a bronze scaphe sundial typically with a diameter larger than 300 mm, particularly at the positions of SPCS, s and w.
To examine the positions of season lines on a portable scaphe sundial, we test for two images: (a) for a real sundial made by
Fig. 9a shows the result for the sundial of
We should keep one thing in mind in comparing the projected season lines of a real sundial with those obtained by calculations. Although there are some distortions in the surface of a dial plate, it does not cause a serious problem to read a season line in a real scaphe sundial if the line was constructed so as to agree with the incident angle of the ray of the sun. When the distortion in the surface is projected into a plane, however, it may cause an error. For this reason, we need to check the degree of the roundness of the surface of the dial plate when using a projected image.
Ratio to the lengths of projected season lines according to the observer’s latitude (when the diameter of a dial plate is 1.000).
Lengths of projected season lines according to the observer’s latitude (when the diameter of a dial plate is 30 mm).
In Fig. 10, we present a digital image of a real scaphe sundial made by
As in Fig. 9, we compared the season lines of the sundial of
[Fig. 10.] The verification of season and hour lines in the scaphe sundial made by Yun Kang in 1880.
In the comparison of projected season lines with respect to the observer’s latitude (36˚~38˚), the difference within the angle of less than 1˚ can be easily distinguished by the position of s rather than by that of sp or w. Nonetheless, it is practically impossible to verify the polar height (i.e., observer’s latitude) used in manufacturing a sundial within an arcminute range. Instead, this 2D projection method will be useful as a simple check, particularly in terms of astronomy, for scaphe sundials that are preserved in a general or science museum. For a more detailed inspection, we believe that 3D measurement is required.
9Treasure no. 845 is divided into 845-1 and 845-2. The former is 35.2 cm in diameter and 14 cm in height, and was manufactured in the later 17th century, while the latter is 24.3 cm in diameter and manufactured around the early 18th century. Both are currently preserved in National Palace Museum of Korea. 10In Fig. 8b-d, we assume the radius as unity. In Table 3, on the other hand, we present values in the ratio of the diameter of unity, for the convenience of conversion. 11Image source: Seoul Museum of History (http://www.museum.seoul.kr)
In this paper, we investigate Sebo of the Kang Family of Jinju, which was closely involved in the manufacture of sundials during the later Joseon dynasty period. Of the 12 sundials made by the Family, 8 are portable scaphe types, which is a unique style that was first developed in the Joseon dynasty, and that was influenced by the western portable plane sundial. In particular, the advent of Daryimdae in Korea’s scape sundials can be understood as a fusion with western technology. Although similar sundials can be found in Japan, they are inferior to Korean sundials in terms of the manufacturing technique and the level of accuracy.
To verify the accuracy of the sundials made by Yun Kang and Geon Kang from an astronomical point of view, we used 2D images of the sundials that were obtained using a photograph and a scanner. In the process of this study, we found that it is important to use the devices of optical tilt and shift in taking a photograph, in order to avoid the distortion of the image. Throughout the analysis for the construction of season lines, we also found that differences of less than 1˚ in the polar height were minor in terms of estimating the accuracy of a portable scaphe sundial. In other words, a portable scaphe sundial can also be used in Gyeonggi-do, a province surrounding Seoul. From this point of view, it can be evaluated that a portable scaphe sundial is a time measurement device that emphasizes practicality and convenience rather than accuracy.
In conclusion, we think that the 2D projection method is very useful for estimating the actual area of a sundial in use, and the latitude applied to its manufacture. We also think that this study is helpful for restoring the damaged season or hour lines, determining the length of the lost