Reading the Heavens Part 3 – The Astronomical Legacy of King Sejong

As mentioned in part 1, King Sejong presided over the zenith of Korean astronomical achievement. The construction of a large observatory at Gyeongbok Palace in 1438 – later destroyed without a trace in the Japanese invasion – played a key role in the country’s progress. On the roof were installed various astronomical instruments such as an armillary sphere (honcheonui, 혼천의), a celestial globe, giving a spherical model of the heavens, (honsang, 혼상), and a gnomon (gyupyo, 규표), a device for measuring the altitude of the sun.

This plethora of technology enabled great accuracy – eclipses could be predicted to the very second of their occurrence, and calendar periods could be calculated correctly to significant figures. Korea was one of only three 15th century nations able to calculate the movements of the planets with respect to its own latitude.

Model of astronomical instrument ‘Kanui’ (간의) at the royal tomb of King Sejong

The observatory was staffed on a daily basis by officials from the Hall of Heavenly Records. When unusual celestial phenomena, such as eclipses or comets, were seen, a report was prepared for immediate dispatch to the Royal Secretariat and other major government offices.

The Confucianist outlook that prevailed in the Joseon dynasty required that the King respond to heavenly omens such as solar or lunar eclipses. The ceremony known as kusikrye was performed in order to interpret the will of the heavens and determine an appropriate course of action. This was part of the wider philosophy of Confucianism, which emphasised the importance of living in harmony with nature, and fulfilling one’s role in the greater cosmic order.

The practicalities of this meant that having an accurate knowledge of when such phenomena would occur was useful from a logistical point of view, as it gave the King time to prepare, and made performing the ceremony on the day a process of precision rather than chance.

As calendrical astronomy was a matter of national importance, King Sejong made exceptions in both rank and procedure in order to appoint the best minds in the kingdom to take charge. He himself studied the Xuanming calendar from Tang China and compared it with the Shoushi calendar from Yuan China, together with many other books related to astronomy and the calendar, in order to produce a calendar of Korea’s own.

In addition, he took detailed observations of solar and lunar eclipses and compared his observations with the forecasts of existing calendars to assess their accuracy. These efforts of Sejong required great patience, as they were made over a long period of time.

Model of armillary sphere (Honcheonui, 혼천의) at the royal tomb of King Sejong
Model of armillary sphere (Honcheonui, 혼천의) at the royal tomb of King Sejong

The fruit of the collective labours of Sejong and his astronomers later appeared with the publication of A Calculation of the Movements of the Seven Celestial Determinants (1442). A landmark in the history of astronomical science, it made it possible for Joseon astronomers to calculate the position of all the planets with respect to Seoul, as well as the times of sunrise and sunset, and solar and lunar eclipses. The book calculates one year to be 365.2425 days, and one month to be 29.530593 days, values correct to six significant figures according to modern calculations.

The work was introduced to Japan by an envoy named Pak In-gi in the 17th century, and used as a basis by Japan to produce its own astronomical calendar. When the book was completed in 1442, however, only Korea, China, and Arabia were able to compute celestial phenomena to such a high degree of accuracy, and with respect to their own latitude.

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