However, their descriptions are insufficient to understand the accuracy of the records and relationship between Chinese and Japanese systems of units of time. Ancient Japanese units of time are described by Hirayama ( 1913a, b) and Hashimoto (1966) in Japanese. Ancient Chinese units of time are described by Stephenson (1997) and Steele (2000). On the other hand, Japanese units of time did not appear in official histories, like Nihongi, but were scattered in various private histories, diaries, temple records, and so on. For studies using ancient astronomical records, such as timed solar eclipses, detailed knowledge on ancient units of time is required.Ī detailed description on the units of time used in China can be found in official Chinese chronicles (the 24 Chronicles). Stephenson, Morrison 1984 Han, Zhang 1996 Stephenson 1997, 2003 Tanikawa, Sôma 2004a, b Kawabata et al. Recently, studies on the historical records of astronomical phenomena by advanced techniques have been highlighted for investigations concerning the variation of the rate of the Earth’s rotation, solar activities, geophysical phenomena, and so on (e.g. Symmetry of the data in tables with respect to certain dates was also investigated in detail in order to examine how the dates of 24 qis were determined. It has been found that all of the daytime and nighttime, the times of sunrise and sunset, and the lengths of shadows given in the official Chinese chronicles refer to a particular latitude of about |$$|, and that the Japanese system adopted this Chinese tradition. In ancient Japan, the time units ke and fen were also used, but the lengths of both of them varied depending on the era. These numbers were clarified by analyzing the tables of daytime duration given in the official Chinese chronicles. The number of fen in 1 ke varied from dynasty to dynasty. One day was also divided into 100 equal parts, called ke, and ke was subdivided by a time unit called fen. However, it is confirmed in this paper that in the Chinese Song dynasty the first double hour began at 0 hr local time. It is well known that both in ancient China and Japan 1 day was divided into 12 double hours, and the first double hour began at 23 hr local time.
The time systems employed in ancient China and Japan are discussed.
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