Astronomer Representing Joseon

Even from the ancient past, people have measured time. They have created calendars on a daily, monthly or yearly basis, and used them in their everyday lives. The calendar we use today is produced based on the almanac that contains days on the solar and lunar calendars, 24 seasonal divisions, the times the sun and moon appear, and the changing phases of the moon.

In the early period of the Joseon kingdom, an almanac from the Ming Dynasty of China was used because there was no Korea-based almanac available. Every autumn, a special envoy known as Dongjisa(동지사) was dispatched to Beijing to bring the Chinese almanac for the following year.

But during the reign of King Sejong, an almanac was created to be applied to farming. Based on the times of sunrise and sunset in Hanyang, which is present-day Seoul, the new calendar reflected the characteristics of Korean land and seasons. The calendar book was called Chiljeongsan(칠정산), meaning the calculation of the motions of the seven heavenly objects.

Joseon’s representative astronomer Yi Soon-ji created Chiljeongsan Oepyeon(칠정산외편), the first Hanyang-based calendar book.

Raising Level of Joseon’s Astronomy to Global Standards

In the Joseon era, the calendar meant a lot more than just days. It also had astrological significance to foresee the security and welfare of the dynasty. Therefore, great efforts were made to observe and calculate the movements of the heavenly bodies precisely. King Sejong, in particular, made a considerable investment in developing astronomy and meteorology.

Yi Soon-ji assisted the king and made a great contribution to the development of astronomy. He is considered an astronomer who helped raise the level of Joseon’s astronomy to global standards.

When speaking of the development of science and technology under King Sejong, most Koreans think of famous scientist Jang Young-sil. But in those days, Yi Soon-ji stood unchallenged in astronomy and calendar development.

The king was frustrated with the inaccurate almanac at that time. He selected talented people out of many civil officials and had them learn arithmetic. It turned out that Yi was the best of the bunch.

The king began to show interest in Yi when the astronomer calculated that the center of the Korean Peninsula was located at 38 degrees north latitude. At first, the king was a bit dubious about the calculation. However, he later saw a calendar from China, and knew that Yi’s computation was exactly correct. From 1431, the king entrusted Yi with the task of carrying out astronomical observations and developing the calendar.

Calendar for Korea

The annals recorded after Yi’s death show that he created various astronomical instruments and facilities to follow King Sejong’s order. This indicates that he played a significant a role in this area.

Yi’s most remarkable achievement is the publication of Chiljeongsan Oepyeon, which is considered the best Arabic astronomical almanac published in Chinese characters. The word Chiljeong refers to seven celestial objects; the sun, the moon, Mercury, Mars, Jupiter, Venus and Saturn. This calendar book describes the motions of the sun and the moon, solar and lunar eclipses and the movements of the five planets. Based on the Greek and Arabic traditions of astronomy, this book states that a year consists of 365 days and there are 31 leap days in every 128 years.

According to the book, a year comprises 365 days, 5 hours, 48 minutes and 45 seconds, to be more exact. This is even more accurate than the traditional Chinese Shoushi Calendar. Compared to today’s calendar, the difference is only one second.

Yi’s calendar book applies the outcome of Arabic astronomy to the situation of Korea. Thanks to the book, Joseon reached the world’s most advanced level in astronomy, along with Arabia and China.

Yi wrote many other astronomy and almanac books. Jegayeoksangjib(제가역상집) published in 1445, for example, is a compilation of diverse astronomical theories from other books. A book entitled Gyosikchubobeop(교식추보법) was published in 1459 to give an easy explanation of how to calculate solar and lunar eclipses. Another book Cheonmunyucho(천문유초) deals with astronomical and meteorological phenomena.