Study on the system of night hours for decoding Korean astronomical records of 1625–1787

This paper presents a study of the dates and times of astrophysical phenomena seen in the night time hours in Korea between 1625 and 1787. This is a period when two different calendars were used and it is important to know which calendar was used to record events such as lunar eclipses. It is known that the Joseon court adopted Shixianli (a Chinese calendar of Adam Schall) in 1654, the fifth reign of King Hyojong. However, the year when the court introduced the calendar into the system of night hours has not yet been determined. To know the enforcement year is very important for studies on astronomical events that are presented in Korean historical documents. From Seungjeongwon Ilgi (Daily Records of the Royal Secretariat), we compile a total of 90 lunar eclipse records referring to the observation time of the eclipses and calculate the times of occurrence of the eclipses with respect to the calendrical methods: Chiljeongsan Naepyeon (a Korean calendar) and Shixianli. As a result, we find that the system of night hours by the former calendrical method was used in the Joseon dynasty until as late as 1710. We also verify that the times of sunrise and sunset were considered as the moments when the center of the Sun reached the horizon according to Chiljeongsan Naepyeon at least. Therefore, we think that this study will contribute to the studies on astronomical phenomena of the Joseon dynasty, particularly on the estimate of the observation time.