Accuracy of Solar Eclipse Predictions in the Chiljeongsan Oepyeon

This study systematically analyzed the accuracy of solar eclipse predictions in the Chiljeongsan Oepyeon (Supplementary Treatise of the Seven Luminaries; hereafter referred to as Oepyeon), which was largely re- constructed based on the system of the Huihui Calendar. Compiled during King Sejong‘s reign, the Oepyeon was used to assist with eclipse calcu- lations in the Shoushi calendar series. However, because the astronomical tables of the Huihui Calendar were used without adjustment for the lat- itude of Hanyang, the intended precision was not fully achieved. Despite this, by adjusting the correction value for the lunar mean longitude, the Oepyeon was able to maintain accuracy nearly identical to that observed in Nanjing. Although the Oepyeon provided more accurate predictions com- pared to other methods at the time, the limitations of the Huihui Calendar could not be overcome with the available astronomical knowledge. Predictions still showed errors of up to several tens of minutes, even after adjustments were made to the lunar mean longitude. This suggests that the Oepyeon’s method had inherent limitations that required further refinement. The Annals of the Joseon Dynasty record the Oepyeon as more accurate than other methods, but given the errors observed in this study, these records may reflect exaggerated assessments based on miscalculations. The relative accuracy of the Oepyeon could also be attrib- uted to the much poorer predictions of alternative methods. While the Oepyeon provided some degree of accuracy, its limitations highlight the scientific challenges faced by Joseon astronomers. This study emphasizes the importance of further research into traditional astronomical methods to better understand their historical and scientific significance.