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.

ABSTRACT
1. 서론
2. 8자 높이의 소규표
    2.1. 8자 규표의 제작
    2.2. 소규표 모델
3. 그림자길이 관측
    3.1. 관측 자료
    3.2. 연중 그림자길이 비교
4. 동지시각의 추정
    4.1. 문헌자료
    4.2. 현대계산 및 관측분석
    4.3. 그림자길이 계산법의 특성
5. 결론
ACKNOWLEDGMENT
REFERENCES
APPENDIX

• 김상혁(한국천문연구원) | SANG HYUK KIM (Korea Astronomy and Space Science Institute, Daejeon 34055, Korea)
• 민병희(한국천문연구원, 충북대학교, 과학기술연합대학원대학) | BYEONG-HEE MIHN (Korea Astronomy and Space Science Institute, Daejeon 34055, Korea, Chungbuk National University, Cheongju 28644, Korea, Korea University of Science and Technology, Daejeon 34113, Korea) Corresponding author
• 유경한(충북대학교, 충북Pro메이커센터) | GYEONG-HAN YOO (Chungbuk National University, Cheongju 28644, Korea, Chungbuk Pro Maker Center, Cheongju 28644, Korea)