The fourth monarch King Sejong (r. 1418-50) of the Joseon dynasty (1392-1910) launched a program to equip the Royal Observatory (Ganui-dae) during 1432-38 to establish Neo-Confucian political ideology, and to promote agriculture and social culture. The project was focused on adjusting the Season-granting system fitting to the latitude of Hanyang (now Seoul). And it was also aimed on making observational equipments for calculating astronomical constants fitting to Hanyang as well as timekeeping devices for civil services. Sejong constructed five kinds of astronomical equipment in the main palace grounds: the Simplified Armilla and its platform, the forty-cheok -high (828 cm) Template and Bronze Tall Gnomon with Shadow Aligner, the Direction-determining Square Table and the Water-operated Armillary Sphere and Celestial Globe. To do this, the Korean version of the ChineseZhou Foot-Rule (Ju-cheok) which is equivalent to 20.7 cm employed as standard measure for scaling. In 1442, the first Joseon native calendar Calculations on the Celestial Motions of Seven Regulators (Chiljeong-san) came into use. Moreover, the Rain-gauge (Cheugu-gi), the oldest known philosophical instrument for measuring precipitation, was invented at this time and put into use. The new system of observing the rule of heaven enabled the king to implement the ruling by Neo-Confucian rites and virtues following the practices of the Yao and Shun. The Observatory provided the momentum to lay the groundwork for scientific and social norm in fifteenth century Korea. Sejong’s achievements not only helped to advance East Asian astronomy, but also set a milestone in the world history of astronomy.

1. 1.

   Needham et al. (1986),HHR, p. 17.

2. 2.

   Kim L (ed) (2004),Koguryo Tomb Murals, p. 15 and 74.

3. 3.

   Nam MH (2008), Time in Korea.

4. 4.

   Rufus WC (1936), p. 18 and Figs. 13 and 22.

5. 5.

   Yi TJ, Jeon SW (1998) Science, Technology, and Agriculture in Fifteenth Century Korea.

6. 6.

   Rufus WC (1913), p. 27.

7. 7.

   Yi TJ (2012), op cit, pp. 277–278.

8. 8.

   Jeon SW (1998),A History of Science in Korea, p. 124; Park SR (1998),Portents and Politics in Korean History, p. 43.

9. 9.

   Ming China used Nanjing-based calendar and time after moving the capital to Beijing until the adoption of Temporal Model Calendar (Shixian-li) in 1656.

10. 10.

    365.2425 days.

11. 11.

    Outer VolumeChiljeong-san oe-pyeon was compiled based on the Chinese-Islamic calendar ( Huihui-lifa) andGateway to the Islamic Method for the Computational Case-study of the Sun ( Weidu Taiyang Tongjing). (See Yu GR2001, p. 43) After the compilation of new calendars, it seems that the Astro-calendric Office made the astronomical almanacs according to theChiljeong-san under the name of the Great Concordance ( Datong-li) which was distributed by the Ming dynasty. (SeeSejong sillok 101: 4a).

12. 12.

    Outer Volume ( Oe-pyeon) proved to be superior to predict eclipses and employed as a supplementary calendar. (Lee EH and Hahn YH (2012),Korean Journal History of Science, Vol. 34, No. 1, pp. 36–70).

13. 13.

    Kim YS (1998), OSIRIS, Vol. 13, p. 56.

14. 14.

    Lee PH and Theodore de Bary W (eds) (1997), Sources of Korean Tradition, Vol. 1, p. 262.

15. 15.

    The Veritable Records of Sejong issued in 1454 where every significant occurrence from the time when King Sejong ascended the throne was entered chronologically by year, month and day in that order. The Observatory Records is an article on the fifteenth day of the fourth lunar moon of the nineteenth year (1437) after ascending to the throne.

16. 16.

    Lee PH and Theodore de Bary W (eds) (1997), op cit, p. 293.

17. 17.

    Sejong sillok 156–158: Inner Volume, 159–163: Outer Volume; Yu GR (2001), pp. 33–57.

18. 18.

    Sitian-tai was called ancient observatoryling-tai where the Guo Shoujing’s Simplified Instrument, Tall Gnomon, Shadow Aligner, Observing Table, Ingenious Instrument, Upward-facing Instrument, Direction-determining Table, Celestial Globe, Star-dial Time-determining Instrument, Lantern Clepsydra at Daming Hall and other instruments were installed. Guo might have employed a sky measuring rulertianwen-chi equivalent to 24.5 cm per onechi, in contrast to King Sejong who employed KoreanJu-cheok ( Zhou-chi in Chinese) 20.7 cm. (Ref. Wang D2011, pp. 300–301) So the scales of the equipment of Sejong’s observatory were smaller than Guo’s observatorysitian-tai.

19. 19.

    Pan N and Xiang Y (1980), pp. 30–39; Pan N (ed) (2005), pp. 157–59; Needham et al. (1986)HHR, pp. 64–68; Sivin N (2009), Granting the Seasons, pp. 194–198, etc.

20. 20.

    Yuan Treatise 1: 1.

21. 21.

    Actually 38.25^d was the polar elevation ofGaeseong the capital of Goryeo dynasty, not Seoul. Thus they made bronze instruments employing 38 1/6^d following Lee Soon-ji. (SeeSejo sillok 36: 14a; Lee and Kim (1458),Calculation of Luni-solar Eclipses).

22. 22.

    In the Season-granting system, the circumference of the circle was 365.25^d (Chinesetu, Koreando) compared with the Islamic calendar’s 360 deg.

23. 23.

    Sejong sillok 80: 26b.

24. 24.

    Needham et al. (1986),HHR, p. 68; Nha IS and FR Stephenson (eds) (1997), Oriental Astronomy from Guo Shoujing to King Sejong.

25. 25.

    On February 13–14, 1993, three Korean Professors Nha Il-seong (an author of this chapter), Park Seong-rae and Nam Moon-hyon (an author of this chapter) had a field trip to Purple Mountain Observatory, Academia Sinica, in Nanjing, China guided by late Professor Xu Zhentao and examined Ming reconstructed Simplified Instrument, Armillary Sphere and Template and Bronze Gnomon, and next year Professor Lee Yong-sam and Nam Moon-hyon had second tour.

26. 26.

    On moving the platform, Ref.Sejong sillok on 3rd, 14th, 22nd of the first lunar moon and 4th of the second lunar moon of the year.

27. 27.

    Pan and Xiang (1980),Guo Shoujing, p. 63.

28. 28.

    According to the records on the length of shadows measured on the winter solstice in 1568, it was 130 years later first made, length of shadow of the Tall Gnomon was 67.52 c and Smaller Gnomon was 14.52 c on the 27th day of the eleventh lunar moon, eighteenth year throne of the King Myeonjong (r. 1546–1567) ( Myeonjong sillok 29). This suggested King Sejong made traditional 8 c Gnomon in the Royal Observatory additionally.

29. 29.

    Hahn et al. (2001), Korean Historical Studies, Vol. 113, pp. 57–83.

30. 30.

    Su (1937), Essentials of the Method for the New Instruments; Needham et al. (1960) Heavenly Clockwork (HC).

31. 31.

    Nha IS (1994),Celestial Globe of the King Sejong.

32. 32.

    According to the recent studies, Guo Shoujing used Astronomical Ruletianwen-chi, onechi equivalent to 24.5 cm to make his instruments at the Yuan observatoryling-tai in Dadu (now Beijing). (Ref. Wang D (2011),Studies History of Natural Sciences, Vol. 30, No. 3, pp. 297–305).

33. 33.

    Nam MH (1995) Korean Water-clocks, pp. 271–304.

34. 34.

    Sejong sillok 107: 21b;Jeungbo munheon bigo 1,Sangwigo 2: 31b.

35. 35.

    Nha I S (1996)Dongbanghak-ji 93: pp. 69–101.

36. 36.

    Sejong sillok 92: 24b; Lee PH and Theodore de Bary W (eds) (1997), op cit, p. 308.

37. 37.

    It was called “reporting the amount of precipitation—the gratitude from the heaven.”

38. 38.

    Seong JD (1818) Treatise on the Astro-calendric Office, Vol. 1, p. 30a.

39. 39.

    For example, “From 28th day to today, rainy or shinny, height of the rain gauge was 0.05 c (1.0 cm).” ( Joonjong sillok 98: 41b), etc.

40. 40.

    Jeon SW (1998), A History of Science in Korea, p. 143.

41. 41.

    Wada (1910) had written a report on the Korean Rain-gauge in French, and sent it to his academic acquaintances in France. In the report, he argued that theCheugu-gi was developed about 200 years before Italian Benedetto Castelli (1578–1643) invented a rain gauge in 1638.

42. 42.

    Sejong sillok 93: 22a.

43. 43.

    Baker D (1998) King Sejong the Great, p. 174.

44. 44.

    Sigimondi C2012, Proc. 1st Int. Conf. Korea Science and Culture Agency, pp. 37–41.

The author wishes to thank the King Sejong Memorial Society, National Palace Museum of Korea, Korea Astronomy & Space Science Institute (KASI), Korea Meteorological Administration (KMA), Purple Mountain Observatory and the Institute for the History of Natural Science of the Academia Sinica, Beijing Ancient Observatory , Scientist Jang Yeong-sil Memorial Society, Research Center for the Korean History of Technology of Konkuk University. The author is grateful to Professors Timothy Atkinson at Seoul University of Foreign Studies and Dr. Yoonkey Nam at Korea Advanced Institute of Science and Technology (KAIST) for reviewing the manuscript.

Authors and Affiliations

1. Konkuk University and Jagyeongnu Research Institute, Seoul, South Korea

   Moon-hyon Nam

2. Yonsei University and The Nha Museum of Astronomy, Yecheon, South Korea

   Il–seong Nha

Corresponding author

Correspondence toMoon-hyon Nam.

Editors and Affiliations

1. Department of Physics, Lille 1 University Science and Technology, Villeneuve d’Ascq, France

   Raffaele Pisano

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