Redefining the Sky of Joseon – The Scientific Brilliance of Chiljeongsan Naeoe-pyeon

(from Volume 156 of the Veritable Records of King Sejong)

During the Goryeo Dynasty, Choi Seongji accompanied King Chungseon on a visit to the Yuan Dynasty in China. There, he obtained a copy of the Shoushili Calendar (授時曆法) and brought it back to Korea, where it was adopted for the first time. However, at the time, local calendar officials only understood how to construct the calendar itself. They lacked the knowledge to calculate solar and lunar eclipses or determine the positions of the five planets—Mercury, Venus, Mars, Jupiter, and Saturn.

In response, King Sejong commissioned Jeong Heumji, Jeong Cho, and Jeong Inji to conduct astronomical research and refine the calendar system. These scholars successfully unraveled the underlying principles. Where their understanding fell short, King Sejong himself, with his keen insight, discerned the correct interpretations.

Moreover, the Joseon court obtained a Chinese text titled Taeeum Taeyang Tonggwe (太陰太陽通軌)—“Unified Orbital Principles of the Moon and the Sun.” As its methods differed slightly from the Shoushili, King Sejong selectively condensed and revised it, designating the resulting work as the “Inner Volume (Nae-pyeon)”.

Additionally, the Huihui Calendar—an Islamic calendar system—was acquired. Sejong ordered Yi Sunji and Kim Dam to analyze and compare it with existing systems. Through this process, they discovered flaws even in the calendrical methods used by official astronomers of the Chinese Ming court.

As a result, King Sejong further refined and systematized the findings, compiling them into the “Outer Volume (Oe-pyeon)”.

Through these efforts, the astronomical and calendrical system of Joseon reached a level of completeness and sophistication unparalleled in its time.

1. Why Did Joseon Create Its Own Calendar System?

The Limits of Imported Calendars and the Need for Scientific Independence

During the late Goryeo and early Joseon periods, Korea primarily used the Shoushi Calendar (授時曆) from China's Yuan Dynasty. Although this system was scientifically advanced for its time, it was geographically mismatched for the Korean Peninsula.

For example, the Shoushi Calendar was calculated based on the longitude and latitude of central China (approx. 112°–116°E, 34°–38°N), while Seoul (Hanyang), the capital of Joseon, lies at 126.9°E. This discrepancy led to significant prediction errors in astronomical events such as solar and lunar eclipses and seasonal changes.

Chiljeongsan Naepyeon and Oepyeon
(Calculations of the Seven Celestial Bodies – Internal and External Volumes)

📌 Case Study: Solar Eclipse Prediction (September 6, 1432)

• Based on the Shoushi Calendar: Approx. 47-minute error in Seoul

• Actual observation: Misaligned with expected timing and visibility

• Result: Disruptions to agricultural planning, state rituals, and celestial observances

Recognizing this issue, King Sejong declared:

“The sky of China and the sky of our land are not the same.
If we do not understand the heavens correctly, we cannot govern the lives of our people.”
– Veritable Records of King Sejong (Sejong Sillok), 1442

Thus began the monumental project to develop a scientifically accurate and geographically appropriate calendar system for Joseon, resulting in the creation of Chiljeongsan Naepyeon (Internal Book) and Oepyeon (External Book).

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2. What Are Chiljeongsan Naepyeon and Oepyeon?

Honcheon-ui (Joseon Armillary Sphere)
An astronomical instrument used in the Joseon Dynasty to observe celestial bodies based on a geocentric model of the universe.

📘 Chiljeongsan Naepyeon (Internal Book)

• Based on Chinese astronomical texts like the Taiyin Taiyang Tonggui and the Shoushi Calendar

• Recalculated and adjusted using local Joseon geography and observations

• Developed by prominent scholars: Jung In-ji, Jeong Cho, Sin Suk-ju, Jeong Hyeon-jo, and Yi Sun-ji

Key Achievements:

• Average synodic month: 29.53059 days
  → Identical to NASA’s modern lunar cycle measurement

• Refined solar (ecliptic) and lunar (white path) motion, including visible planetary orbits

• Applied Seoul-based solar time (UTC+8:22:45)
  → Approx. 37 minutes more accurate than the Shoushi Calendar

📕 Chiljeongsan Oepyeon (External Book)

• Incorporated data from the Islamic Huihui Calendar, renowned for its precision

• Translated and interpreted by Yi Sun-ji and Kim Dam

• Used to verify and correct the inaccuracies of traditional Chinese systems

Category	Shoushi Calendar	Chiljeongsan (Adjusted)	Modern NASA Value
Synodic Month (days)	29.53086	29.53059	29.53059
Winter Solstice Error	±30 minutes	Within ±5 minutes	-

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3. More Than a Calendar – A Fusion of Eastern and Western Science

The Daetongnyeok (大統曆), the oldest calendar system used in the early Joseon Dynasty, originally based on the Yuan dynasty's Shoushili calendar.

Chiljeongsan was not merely a calendar but a fusion of multiple global scientific systems, reconstructed through the lens of Joseon’s own scientific methods.

Origin	Content	Role in Chiljeongsan
Yuan’s Shoushi	Traditional astronomical base	Structural foundation (Naepyeon)
Ming’s Tonggui	Latest Chinese calculations	Reference for refinements
Islamic Huihui	Trigonometry, precision tools	High-precision corrections (Oepyeon)

Joseon did not merely adopt foreign knowledge;
it critically integrated and reconstructed global astronomical data, asserting a unique scientific identity.

🧭 By comparison:

• Gregorian Calendar reform (Europe): 1582

• Chiljeongsan Completion: 1442
  → Joseon’s calendar reforms predated Europe’s by 140 years, and were based on observational precision.

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4. Scientific Achievements of Chiljeongsan

Joseon’s astronomical calculations extended far beyond mere observation—they were based on rigorous mathematical modeling.

✔ Application of Trigonometry

• Incorporated sine approximations via the Islamic system

• Enabled accurate calculations of latitude, solar elevation, and seasonal shifts

✔ Systematic Error Correction

• Achieved ±5 minute error margin for Seoul

• Maintained 95%+ accuracy for predicting eclipses and solar terms

✔ Integration of Theory and Observation

• King Sejong ordered the creation of observational instruments such as the armillary sphere (ganui) and sundials (angbu-ilgu)

• Yi Sun-ji and others used this data to develop correction algorithms for calendar predictions

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5. Why Chiljeongsan Still Matters Today

Chiljeongsan Naeoepyeon is not just an ancient almanac—it is a landmark in the history of scientific sovereignty. It demonstrates how 15th-century Joseon embraced and redefined global knowledge to create a culturally and geographically independent scientific system.

Modern Significance	Explanation
Scientific Accuracy	One of the most precise calendars of its time, NASA-level precision
Intellectual Synthesis	Harmonized Chinese, Islamic, and local Korean science
Knowledge Sovereignty	Moved beyond dependency on foreign systems
Cultural Heritage	Deserves global recognition in the history of astronomy

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📚 References (For Academic Reliability and SEO Trust)

1. Veritable Records of King Sejong (Sejong Sillok), 1442

2. Chiljeongsan Naeoepyeon Manuscripts – National Library of Korea

3. Kim Young-sik, History of Science and Technology in Joseon, Science Books

4. Park Min-je, “A Study on Joseon’s Astronomical Independence”, Journal of East Asian Science History, 2020

5. NASA Goddard Space Flight Center – Synodic Month Data

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✍️ Conclusion – “To Understand the Sky Is to Prepare for the Future”

15th-century Joseon looked to the heavens to define its destiny.
Chiljeongsan was the result—a scientifically robust, geopolitically independent calendar system that stood among the best in the world.