[News Terms] 'Wonguilyeong' That Clarified Restoration and Operation Principles

'Portable Sundial' That Can Measure Time Anywhere
Purchased at US Auction, Recovered and Restored, Operating Principle Identified After 133 Years

'Wonguilyeong (圓球日影)' is a "portable sundial" shaped like a round ball (wongu) resembling a globe, with a diameter of 11.2 cm and an overall height of about 23.8 cm, capable of measuring time anywhere by adjusting various devices.

Designated Treasure No. 845, it bears the name "Wonguilyeong (圓球日影)" engraved in intaglio on the upper hemisphere, just as the "Angbuilgu (仰釜日晷)"?a sundial (日晷) shaped like a cauldron (釜) looking up at the sky, made by Jang Yeong-sil and others during King Sejong's reign?has its name inscribed.

A unique sundial from the late Joseon period called 'Wonguilyeong (圓球日影)', designed in a round ball shape to measure time anywhere.
Photo by National Science Museum
Image source: Yonhap News

The two hemispheres interlock, allowing the adjustment of various devices so that time can be read anywhere by measuring the time indicated on the surface. However, some parts were missing or broken, making it difficult to accurately determine how to measure time or operate the device.

Last March, the Cultural Heritage Administration and the Overseas Korean Cultural Heritage Foundation purchased and repatriated this scientific cultural asset from an auction in the United States. It is said to have been made in 1890 by Sang Jikhyeon (尙稷鉉), who served as First Rank Councilor of the Jungchuwon (中樞院) during the late Joseon period. At the time of auction, it was listed under the name "Ilyeong Wongu (日影圓球)," which was later determined to be a misreading of Wonguilyeong.

Thanks to its advantage of measuring time anywhere, Wonguilyeong is considered a more advanced sundial than the fixed-location Angbuilgu. However, it cannot compare to the value of Angbuilgu, the "public sundial," which embodies King Sejong's benevolent spirit by depicting the 12 zodiac animals as pictures rather than characters for the common people who could not read.

The sundial 'Angbuilgu' made during the reign of King Sejong. The 12 zodiac animals were engraved as pictures for the common people who could not read.
[Photo by Wikipedia]

On the 19th, the National Science Museum announced that, in cooperation with the Cultural Heritage Administration and the National Palace Museum, they restored Wonguilyeong and clarified its operating principle after 133 years.

The research team at the National Science Museum restored Wonguilyeong and discovered that, unlike traditional sundials fixed in one location, it uses a method that adjusts the horizontal level and the local latitude (polar altitude) according to the observation point. They also confirmed that it is a portable sundial where the T-shaped gnomon shadow device is aligned to enter the long groove in the southern hemisphere, and the time is read by the northern hemisphere time markings indicated by the tip of the gnomon.

Wonguilyeong consists of a spherical sundial rotating around the north-south polar axis, a polar altitude adjustment device, a supporting pillar, and a base. Time markings are inscribed around the upper hemisphere. The 12 zodiac signs (십이지, 十二支) are engraved, and each hour is divided into two halves called cho (初) and jeong (正), which are further divided into four quarters, making eight segments per hour. One si (시) equals 2 hours in modern time, cho and jeong each represent 1 hour, and each segment (각, 刻) corresponds to 15 minutes.

As a result of analyzing the two lines marked on the artifact's latitude adjustment device (the hook-like protruding part), it was found that the most frequently used area was indicated based on Seoul.
[Photo by National Science Museum] [Image source=Yonhap News]

Only nine of the twelve zodiac signs, from In (寅) to Sul (戌), are engraved in intaglio on the upper hemisphere; Hae (亥), Ja (子), and Chuk (丑) are absent. The research team analyzed this as following the tradition of Angbuilgu by not marking nighttime hours, which cannot be measured by a sundial.

To clarify Wonguilyeong’s operating principle, the research team conducted time measurement experiments with the restored model at locations including Jeju, Daejeon, and Gyeongbokgung Palace in Seoul. By analyzing two lines marked on the latitude adjustment device of the artifact, they found that the most frequently used region was based on Seoul. The time measurement experiments with the restored model showed an error margin within 7 minutes and 30 seconds.

This research involved Yoon Yong-hyun, head of the National Science Museum, Dr. Kim Sang-hyuk and Dr. Min Byung-hee from the Korea Astronomy and Space Science Institute, former Chungbuk National University professor Lee Yong-sam, and Gil Ki-ho, director of the Cultural Heritage Research Institute.