Astronomical data making such as forming a calendar, period of day, determining the time of rising/setting of the sun and the onset of twilight are essential in our daily lives. Knowing the calendar day of the past is particularly crucial for studying the history of a clan or a nation. To verify previous studies in the calendar day of the Joseon dynasty (1392 – 1910), we investigate the sexagenary cycle of the new moon day (i.e., the first day in a lunar month) by using sources such as results of the calculations using the Datong calendar (a Chinese Calendar of the Ming Dynasty) and the data of Baekjungryeok (a Perpetual Calendar; literally, a one hundred-year almanac). Compared with the study of Ahn et al., we find that as many as 17 sexagenary cycles show discrepancies. In the cases of nine discrepancies, we find that the sexagenary cycles of this study are identical to those of the almanacs at that time. In addition, we study five sexagenary cycles by using the historical accounts of Joseon Wangjo Sillok (Annals of the Joseon Dynasty), Seungjeongwon Ilgi (Daily Reports of Royal Secretariat), Chungung Ilgi (Logs of Crown Prince), and so forth. For the remaining discrepancies, we present historical literature supporting the results of this study. This study will greatly contribute to the identification of the lunisolar calendar days during the Joseon dynasty as the dates of the modern (i.e., Gregorian) calendar.

ABSTRACT
1. INTRODUCTION
2. METHODOLOGY
3. ARGUMENT
    3.1 March 1397, May 1399, and December 1440
    3.2 April 1430 and December 1444
    3.3 February 1530 and September 1597
    3.4 September 1602
    3.5 September and October 1652, and October 1677
    3.6 November 1698 and February 1709
    3.7 Intercalary March 1727 and April 1730
    3.8 November 1751
    3.9 April 1778
4. SUMMARY
REFERENCES

• KI-WON LEE(Catholic University of Daegu) Corresponding Author
• YOUNG SOOK AHN(Korea Astronomy and Space Science Institute)
• BYEONG-HEE MIHN(Korea Astronomy and Space Science Institute, Department of Astronomy and Space Science, Chungbuk National University)