Japanese astronomy in the seventh century

Abstract

There are astronomical records in both the a and b volumes of the Nihongi, and we have already shown that the reliability of these records depends on which particular volumes they appeared in. In order to strengthen and extend our previous conclusion, we study more thoroughly the astronomical data in the Nihongi and analyze the reliability more precisely with reference to Chinese and Korean history books. There are only three astronomical records in the volumes of the a group, and none of these can be said to be observational. In the volumes of the b group, there is, in each of the three volumes, one record, which was surely based on actual observations. Five records of comets are common to Chinese records, but wording and the form of the records are different in Japanese and in the continental records, so the records are judged not to have been transported from China or Korea. Most of the remaining records represent local phenomena, which we believe are all based on observation. In the case of eclipse records, we not only analyze the reliability of each event but also apply elementary statistics to weather records in order to examine the completeness of the records. We conclude that there were major changes in observational astronomy in Japan during the seventh century. Specifically, observations were made during AD 620–641 and AD 672–686 (in the b group of the Nihongi), but no observations were made between AD 642 and 671 (in the a group). After AD 686 (during the Jitô era), no observations were made except for one appulse observation of Mars and Jupiter.