『교식추보법가령』 연구

Abstract

Soon after the compilation of Chiljeongsan-naepyon (七政算內篇) in 1444, some parts of Joseon's first domestic calendar system were revised and improved during the reign of King Sejong. Several arithmetic formulae were modified to make the prediction of the eclipse simple and convenient. The frequent and onerous recourses to the bulky ready reckoning tables were phased out of the calculation procedure owing to the new formulae invented by Joseon mathematical astronomers. All the contents of improvement were reflected in the Book of Explanations and Exercises for Eclipse Prediction, Gyosikchubobeob Garyong (交食推步法假令), which was published in 1458, the fourth year of King Sejo’s reign. The first part of this article reports on the details and effectiveness of the newly devised arithmetic to calculate the following six items: the expansion/contraction difference of the sun, the slackening/hastening difference of the moon, the addition/subtraction difference for eclipse correction, the sunrise/sunset parts of a day, the corrected extent motion degrees at the eclipse maximum, and the lodge positions on the Red/Yellow Way. The rest of the article deals with the meaning and the origin of the newly defined formula for addition/subtraction difference used to obtain the corrected time of the conjunction and the full moon. It attracts remarkable attention because the angular speed of the moon relative to the sun was considered for the first time in the long history of East Asian calendars, in the denominator of the formula for the addition/subtraction difference for eclipse correction. Moreover, some concern was paid to explain how and why King Sejong’s point of view on the results of eclipse prediction had changed generously during the progress of establishing his own calendar from Chinese systems.