Since the division of the Korean Peninsula in 1948, South and North Korea have independently developed their astronomical almanacs: Ryeokseo at the Korea Astronomy and Space Science Institute in South Korea and Cheonmunryeok at the Pyongyang Astronomical Observatory in North Korea. This study compares Ryeokseo and Cheonmunryeok for the year 2015, focusing on publication systems, content, terminology, and differences in data calculation methods. Additionally, it examines the calendars of South and North Korea from 2018 to 2023, analyzing similarities and differences in the representation of calendrical dates, public holidays, and other related aspects. The findings reveal that while the structure and content of the astronomical almanacs are similar in both countries, notable variances exist in the versions of ephemerides, time scales, and calculation precision. Consequently, identical data points are often recorded with slightly different values in each country's almanacs. Furthermore, approximately 28% of the terms used in North Korea's astronomical almanac are either not utilized in South Korea or have different definitions. Regarding calendar systems, those of South and North Korea are largely similar, resulting in no significant discrepancies in dates. However, there are notable differences in the observance of public holidays. While traditional holidays are common to both, most holidays are distinctively celebrated. Notably, North Korea does not observe religious holidays, and many of its holidays are associated with the regime.

In order to encourage collaboration with North Korea in astronomy research field, we have studied the astronomical almanacs published in South and North Korea. The almanac contains fundamental astronomical data including not only daily calendar data but also unique characteristics selected by necessity in each country. We compared the South and North Korean astronomical almanacs in terms of contents, detailed descriptions, unique items, and so forth. We used the 2017 almanac for South Korea and 1993 almanac which is useful for this research for North Korea. We found that there were several differences between South and North Korean almanacs. The North Korean almanac is published for the astronomer or navigator, thus it has supplementary information about spherical astronomy such as precession of the north pole and position of celestial bodies. Whereas the South Korean almanac is published for the general public, and it distinctively contains luni-solar calendar, 24 solar terms, national holidays, etc. This study could be helpful for research cooperation between South and North Korea in astronomy.

The royal astronomical observatory compiled the Astronomical Almanac during the Joseon dynasty, though there were some changes of its organization. However, the observatory underwent sudden changes in the late period mainly due to the influence of historical events such as the Gabo (甲午) and The Eulmi (乙未) Reforms in 1894 and 1895, respectively, and the Japanese invasion in 1910. In this paper, we study the changes of the compilation institution of the Korean Astronomical Almanac and of its organization for the period of 1894 to 1912. During this period, the name of the observatory had been changed several times, from Gwansanggam (觀象監) to Gwansangguk (觀象局) in 1894 and to Gwansangso (觀象所) in 1895. In addition, the affair of the Astronomical Almanac compilation was transferred to the Editorial Bureau [編輯局] of the Ministry of Education [學部] and to the Editing Department [編輯課] of the Governor-General of Korea [朝鮮總督府]. In 1907, the Gwansangso was abolished. Moreover, the affair of timekeeping was separated and the official number of personnel was reduced to less than 5% compared to that of Gwansanggam. Consequently, the royal astronomical observatory was significantly reduced in terms of its functions and the organization through the process of those changes. Therefore, we believe that this period is important when seeking to understand the transition between the traditional Astronomical Almanac of the Joseon dynasty and its modern astronomical counterpart of the present day.

During the Joseon dynasty, the Gwansanggam (觀象監, The Royal Astronomical Observatory) was the royal administration in charge of Astronomy (天文), Geography (地理), Calendars (曆), Astronomical and meteorological observation (測候), and Timekeeping (刻漏). Of those affairs, the Astronomical Almanac compilation was most important. In this paper, we study the education and selection of astronomical officials in the 19th century during the Joseon dynasty, focusing on Astronomical Almanac officials. According to this study, their main area was calendar calculation, and they were involved in Astronomical Almanac compilation for nearly 40 years, until they resigned. In addition, they served as Samryeok-susulgwan (三曆 修述官, Official for calendar calculation by the Shíxiàn calendar), and Ilgwa-gamingwan (日課監印官, Official printing supervisor). To undertake these core duties, an official had to pass several examinations and courses. The Gwansanggam (after Gwansangso) carried out the educational function for the officials. In particular, reserved officials of the astronomy division had to be educated starting with a trainee course.

Korea Astronomy and Space Science Institute (KASI), direct decendant of Korea National Astronomy Observatory, has been publishing Korean Astronomical Almanac since in 1976. The almanac contains essential data in our daily lives such as the times of sunrise, sunset, moonrise, and moonset, conversion tables between luni-solar and solar calendars, and so forth. So, we are planning to register Korean astronomical almanac data for national Standard Reference Data(SRD), which is a scientific/technical data whose the reliablity and the accuracy are authorized by scientific analysis and evalution. To be certificated as national SRD, reference data has to satisfy several criteria such as traceability, consistency, uncertainty, and so on. Based on similarity among calculation processes, we classified astronomical almanac data into three groups: Class I, II, and III. We are planning to register them for national SRD in consecutive order. In this study, we analyzed Class I data which is aimed to register in 2009, and presented the results. Firstly, we found that the traceability and the consistency can be ensured by the usage of NASA/JPL DE405 ephemeris and by the comparsion with international data, respectively. To evaluate uncertainty in Class I data, we solved the mathematical model and determined the factors influencing the calculations. As a result, we found that the atmospheric refraction is the main factor and leads to a variation of ±16 seconds in the times of sunrise and sunset. We also briefly review the histories of astronomical almanac data and of standard reference data in Korea.

We have calculated the astronomical almanac 1994 and simulated the trajectory of a satellite orbit considering all perturbative forces with various initial conditions. In this work, Gauss Jackson multistep integration method has been used to calculate our basic equation of motion with high numerical accuracy. It has beer. found that our results agree well with the Astronomical Almanac Data distributed by JPL of NASA and the orbit simulations have been carried out with fast speed, stability and excellent round-off error accumulation, comparing with other numerical methods. In order to be carried out our works on almanac and orbit calculations easily by anyone who uses a personal computer, we have made a computer program on graphical user interface to provide various menus for detail works selected by a mouse.