Reliability of Eclipse Records in Japanese Ancient Periods
Mitsuru Sôma and Kiyotaka Tanikawa
National Astronomical Observatory of Japan,Mitaka, Tokyo 181-8588, Japan.
E-mail: mitsuru.soma@nao.ac.jp, tanikawa.ky@nao.ac.jp
ththAbstract: Eclipse records began in the 7 century in Japan. Eclipses in the 7 century were recorded inth
the Nihongi and their reliability changed according to the volumes. Eclipses in the 8 century wererecorded in the Shoku-Nihongi but most of them were written according to the predictions at the time.
ththEclipses between the 9 and 11 centuries were recorded in various literatures and diaries but it is noteasy to decide whether they were based on actual observations or not.
Keywords: Earth’s Rotation, Eclipses, Ancient Records

1. INTRODUCTION
Ancient total or annular solar eclipse observations provide the values of ΔT = TT －UT, where TT is
Terrestrial Time, which is a uniform measure of time, and UT is Universal Time, which is determined by
the rotation of the Earth. Hence we can investigate how the past Earth’s rotation speed varied with timefrom such observations (see Stephenson 1997, Tanikawa & Sôma 2004, Sôma et al. 2004, Tanikawa etal. 2010). However there are many eclipse records based on predictions at the time and it is not always
evident if the records were based on actual observations or not. Therefore for our investigations of thepast Earth’s rotation speed we need to distinguish the actual eclipse records from predicted ones.
In this paper we report the results of our investigations about the reliability of Japanese eclipsethrecords until the 11 century. The dates are given here in the Gregorian Calendar although the actually
recorded dates in those days in Japan were in luni-solar calendars.

2. ECLIPSE RECORDS IN THE NIHONGI
Ancient Japanese astronomical records of aurorae, eclipses, comets, meteors, occultations and appulsesof planets were given in Rikkokushi (六国史, Six Formal Japanese History Books, see Table 1). The
Japanese astronomical records began in AD 620 in the Nihongi (Chronicle of Japan) and the first eclipse
record was that of the total solar eclipse in AD 628. In total there are 11 solar eclipses and 2 lunareclipses in the Nihongi.
Table 1. 六国史 Six Formal Japanese History Books
Name of the Book
PeriodCompleted
720
日本紀(日本書紀) Nihongi－ 697
797
続日本紀 Shoku-Nihongi
697 － 791
840
日本後紀 Nihon-Kôki
792 － 833
869
続日本後紀 Shoku-Nihon- Kôki
833 － 850
879
日本文徳天皇実録 Nihon-Montoku-Tennô-Jitsuroku
850 － 858901
日本三代実録 Nihon-Sandai-Jitsuroku
858 － 887
The Nihongi was written in Chinese characters and the volumes of the Nihongi were classified into
the α group and the β group according to the usage of Chinese characters (Mori 1991, 1999). Thevolumes in the α group were written using correct Chinese characters whereas those in the β group
had improper Chinese characters. This implies that the volumes in the α group were written by people
from China and those in the β group were written by Japanese scholars. The periods of the α and βthgroups and the names of the emperors or empresses of the periods in the 7 century are given in Table
2. Tanikawa & Sôma (2008) and Sôma & Tanikawa (2011) showed that astronomical records in the βgroup are reliable whereas those in the α group are not reliable. For example the total solar eclipse on628 April 10 and the lunar occultation of Mars on 681 November 3 are recorded in the β group of theNihongi and they were actually occurred (for their reliability see Sôma et al. 2004), but a lunar occultationof a star on 642 August 9 and a lunar eclipse on 643 June 8 recorded in the α group of the Nihongi
1should not have been actually seen in Japan. A lunar occultation of the first magnitude star Aldebaranon 640 March 3 is recorded in the β group of the Nihongi and it is also shown that it was the real event
although it was only 2-3 years before the false records in 642-643. All the astronomical records in theNihongi including eclipses are given in the paper by Tanikawa & Sôma (2008).
Table 2. The α and β groups of Nihongi
for the period after AD 593
GroupAD Years
Emperor
β593－628
Suiko 推古*629－641
Jomei 舒明α642－645
Kôgyoku 皇極*645－654
Kôtoku 孝徳
655－661
Saimei 斉明*
661－671
Tenji 天智β672－686
Tenmu 天武－686－697
Jitô 持統* These were empresses. Kôgyoku and Saimei
were the same empress with different names.
It should be noted that the volume of the Jitô era of the Nihongi could not be classified into either αor β. There are 6 records of solar eclipses in only 6 years 691－696 in the Jitô era of the Nihongi but
most of them are unobservable in Japan and therefore the eclipse records in the Jitô era of the Nihongi
are not reliable.

2 ECLIPSE RECORDS IN THE SHOKU-NIHONGIThe Shoku-Nihongi (続日本紀) is the second Formal Japanese History Book, which describes events
between 697 and 791. There are many records of solar eclipses in the Shoku-Nihongi and the first 10
dates of those eclipses are: 698 Aug 12, 698 Dec 8, 699 Nov 27, 701 May 12, 702 Sept 26, 704 Mar 10,706 July 14, 707 Jan 8, 707 Dec 29, and 708 Dec 17, but only 3 of them were observable in Japan (thesolar eclipse on 702 Sept 26 was near total in Kyushu; the solar eclipse on 707 Dec 29 was partial in thewestern Japan; the solar eclipse on 708 Dec 17 was a shallow partial eclipse in the eastern Japan) andother 7 events were not observable in Japan. Especially the solar eclipse on 698 Aug 12 was notobservable anywhere in the World. In addition to them there should have been a total solar eclipse in thenorth-eastern Japan on 700 May 23, and there should have been a partial solar eclipse in Kyushu on 703
Mar 22, but neither of them are given in the Shoku-Nihongi. Therefore it is concluded that the eclipse
records in the Shoku-Nihongi are not reliable.
Other astronomical records in the Shoku-Nihongi include the following:
A conjunction of the Moon with a star at night of 723 Dec 28 (the star should be Venus and the actually
this event was seen in the morning sky).
A conjunction of the Moon with Mars on 724 May 15 (actually the conjunction occurred 2 days later).
A conjunction of the Moon with Saturn at night on 725 Feb 20 (actually the conjunction occurred 6 dayslater).
As noted in the parentheses it is doubtful that these statements were based on actual observations. In
summary astronomical records given in the Shoku-Nihongi are not reliable.

3 OTHER JAPANESE ECLIPSE RECORDS BETWEEN THE 9TH AND THE 11TH CENTURIES
Other eclipse records have not been systematically investigated yet. Here only a few cases are noted.
A record for the solar eclipse on 873 July 28 is given in the Nihon-Sandai-Jitsuroku (日本三代実録),
which is the last of the Six Formal Japanese History Books. The record says, “秋七月癸亥朔, 日蝕無光,虧昃如月初生, 自午至未乃復” which means “In Autumn on the first day of the 7th month there was a solar
eclipse, and there was no light. The eclipsed sun was like a new-born moon in the west. The eclipse
began in the 午 (Uma in Japanese, Wu in Chinese) hour (11－13h) and ended in the 未 (Hitsuji inJapanese, Wei in Chinese) hour (13－15h).” Almost all of the eclipses recorded in this era werepredicted ones and the records usually said “the sun was eclipsed” only, but this record has detailed
descriptions, which suggest that the record was written from an actual observation. We know that the2eclipse was seen as an annular eclipse somewhere in or near Japan and the article suggests that theeclipse was annular in Kyoto. The same eclipse was also recorded in Nishapur, Iran, and the record alsosuggested that the eclipse was annular in Nishapur. The reliability of these eclipse records werediscussed by Tanikawa & Sôma (2004).
For the solar eclipse on 878 October 29 the Nihon-Sandai-Jitsuroku says, “the maximum of theeclipse was at “戌 4 刻 1 分” and the magnitude was 13 半強/15”. The character 戌 means the time 19－21h. The 2-hour interval was divided into 刻 (koku in Japanese, ke in Chinese) and 分 (bu or hun inJapanese, fen in Chinese). There were several systems in which each 2-hour interval had different
numbers of 刻 and 分 and we are not sure what system was used in the description in the above record.
半強 means more than a half. As shown in Fig. 1 the solar eclipse on 878 October 29 should not havebeen seen in Japan and therefore it is apparent that this record was not based on actual observations but
was based on some prediction. The
calendar system used in Japan at thattime was Senmyô-reki ( 宣 明 暦 ,Xuanming-li in Chinese).
How tocalculate Senmyô-reki was written by
Ando (1663, 1676). If one calculates
the 878 October 29 solar eclipse by
Senmyô-reki, the maximum of theeclipse is obtained to be at the time 戌
8 刻 16 分 , and the maximum
magnitude is 12 半強/15. In the case of
Senmyô-rekithe2-hourintervalconsists of 8 刻 28 分 and 刻 consistsof 84 分 and therefore 1 day is equal to100 刻. The differences between therecord and Senmyô-reki’s prediction inthe times (戌 4 刻 1 分 vs 戌 8 刻 16 分)
and magnitudes (13 半強 vs 12 半強 outof 15) suggest that the prediction wasmade by other methods or they mademistakes in calculating the prediction.
The difference in the times of themaximum eclipse might be due todifferent systems of the division of the2-hour intervals.
Chôya-Gunsai (朝野群載) is a collection of literary works during the late Heian Period (平安時代) . It
was compiled by Miyosi no Tameyasu (三善為康; born in 1049 and died in 1139), who was an aristocrat
at that time, and it was completed in 1116. It has a record of the solar eclipse on 975 Aug 10. It says,“the beginning of the eclipse was at the time ‘卯 1 刻 3 分’, the maximum at ‘辰 2 刻 1 分’, and the end at‘巳 0 刻 2 分’. The maximum magnitude of the eclipse was 4/15.” The characters 卯, 辰, and 巳 mean
the times 5－7h, 7－9h, and 9－11h, respectively. Nihon-Kiryaku (日本紀略) is a history book written inthe late Heian Era. The editor and the completion year of Nihon-Kiryaku are not known but it serves as asupplementary collection to the Six Formal Japanese History Books. Nihon-Kiryaku says, “There was asolar eclipse. Someone said that the magnitude was 11/15 but some other person said that the Sun wastotally eclipsed at 卯 or 辰 (which means 5-9h), there was inky blackness, and no light, birds hurled, andstars were seen.” From the concrete descriptions it is highly probably that this was total in Kyoto and ourcalculations show that it was actually total in Kyoto (Tanikawa & Sôma, 2004). Note that this eclipse canbe calculated by Senmyô-reki as follows: the eclipse begins at 卯 5 刻 12 分, maximum at 辰 2 刻 23 分,end at 辰 7 刻 62 分, magnitude 9 半強/15.
Shôyuki (小右記) is a diary written by Fujiwara no Sanesuke (藤原実資; born in 957 and died in1046) who became an official in the mid-Heian period. It has a record of the lunar eclipse on 1031 Aug 5.
It says, “There was a total lunar eclipse. The beginning of the eclipse was at the time ‘酉 7 刻 50 分’, themaximum at ‘亥 0 刻 32 分’, and the end at ‘子 1 刻 42 分’.” The characters 酉, 亥, and 子 mean the times
17－19h, 21－23h, and 23－1h, respectively. The actual eclipse times at Kyoto should have been a few
hours later, but the recorded times are exactly the same as those calculated by Senmyô-reki. Therefore
we can see that the time system Fujiwara no Sanesuke used for this eclipse record was followed by therule that the 2-hour interval consists of 8 刻 28 分 and 刻 consists of 84 分.
34 CONCLUSION
Finding actual eclipse records in ancient times are important to determine the Earth’s rotation rate in thepast. Many eclipses were recorded in ancient times in the world, but they were not always based on
actual observations and it is not always clear if they were based on actual observations. Therefore it isimportant to find eclipse records based on actual observations.
ththEclipse records began in the 7 century in Japan. The reliabilities of eclipse records between 7thth
and 11 centuries were discussed in this paper. Eclipses in the 7 century were recorded in the Nihongi
and their reliability changed according to the volumes, namely the records in the volumes in the β groupwere based on actual observations whereas those in the α group were based on predictions at the time.
thEclipses in the 8 century were recorded in the Shoku-Nihongi but most of them were written accordingthth
to the predictions at the time. Eclipses between the 9 and 11 centuries were recorded in various
literatures and diaries but it is not easy to decide whether they were based on actual observations or not.

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