Akira OKAZAKI Department of Science Education, Gunma University, Aramaki, Maebashi, Gunma 371-8510, Japan
Abstract
We present the latest version of the lunar eclipse list (55 records in total) in Dai Viet Sů Ků Toàn Thư (TT, ), one of the most important historical sources in Vietnam. Keeping in mind that the two Chinese characters "  " and "  ", which are similar to one another in form, have been occasionally confused when they are used for the eclipse, we investigated all the eclipse records including solar ones. We examined such confusion as well as contradiction in the described date found in the records to resolve them as much as possible. We discuss the reliability of the lunar eclipse records in TT by comparing with the results of astronomical calculation. We also briefly discuss the day start time considered by Vietnamese people in those days.
1.
Introduction
It is well known that many astronomical records are found in Chinese, Korean and Japanese historical sources. Comparatively little is known, however, about Vietnamese historical astronomical records. There are a few important Vietnamese historical sources, such as Dai Viet Sử Kỷ Toàn Thư (TT, , Complete Volumes of the Historical Memoirs of Great Vietnam) and Viet Sů Luoc (VSL, ). The former is the official historical book compiled by royal historians of Lê ) in 15th through 17th centuries, and the latter, perished in Vietnam but conserved dynasty ( in a few Chinese library series, is believed to be published much earlier. Ho Peng-Yoke (1964) listed the natural phenomena (including many astronomical ones) recorded found in TT. Okazaki and Yokoo (1983) also surveyed the astronomical records in VSL.
2.
Lunar eclipse records
Concerning lunar eclipse, a number of records are found in TT while none in VSL. Ho Peng-Yoke (1964) listed 34 lunar eclipse records in TT from ancient times through AD1675. Recently, Tanokura and Okazaki (2011) surveyed solar and lunar eclipse records by referring to the collated version of TT by Chen Chingho (1984-86). His collated version contains, besides TT itself, another historical source entitled Dai Viet Su Ky Toan Thu Tuc Bien ( , Supplementary Volumes of TT), which covers the period up to AD1789. From their survey, Tanokura and Okazaki (2011) listed 52 lunar eclipse records. Although their list lacks two lunar eclipse records that should be included, their study conduced to large increase of the number of known Vietnamese lunar eclipse records. For convenience of later discussion, we present the latest version of the list of lunar eclipse records in TT as Table 1 at the end of this note. The table presents 55 records in total, including the two records (Nos.1A and 2A) that are lacking in Tanokura and Okazaki's (2011) list and also one eclipse (No.5A) that is mentioned in an episode of a royal astronomer in 15th century. The latter record will be discussed in the next section. The first column of the table gives sequential number of the lunar eclipse record. For the records except Nos. 1A, 2A and 5A, we assigned number identical to that in Tanokura and Okazaki's (2011) list to avoid confusion in fiture studies.. The second column presents a passage of the record in original text of TT. The third and the fourth columns yield the date (in the format of yyyy mm.dd ) described in the lunar eclipse record and the date at mid-eclipse in Hanoi determined by astronomical calcula- tion, respectively. The described date has been converted to the Julian (before 1582) or Gregorian Calendar date by using Lê Thành Lân’s (2007) table. It is noted that before AD 1544 his table substantially gives the Chinese calendar only because no local Vietnamese calendar in those days is available. For the described date in the third column, in the case of a little uncertain date like "the full moon day (   )" given in the record, we obtained the date by simply assuming that it would be "15th day" in luni-solar calendar, and showed it in parenthesis. For the calculated date in the fourth column, we adopted the date in Universal Time (UT) rather than the date in Hanoi Local Time because date (UT) remains constant throughout the night, and this will be convenient for later discussion. Calculations were made with the astronomical simulation software Stellar Navigator ver.6 (AstroArts Inc). The fifth column shows symbols representing how the eclipses are observed in Hanoi judging from astronomical calculation: Symbols and mean a partial and a total eclipse observed in Hanoi, no eclipse observed in Hanoi though the date is not incorrect, and symbol respectively, symbol no eclipse occurred on any days of the month described in the record. We will discuss a possible one day shift in the date in the next section. The sixth and the seventh columns give the corresponding Oppolzer (1887) number and remarks, respectively.
3.
Discussion
3.1.  Confusion between "  " and "
"
The collated version of TT by Chen Chingho (1984-86), to which our study has referred, remarks on the difference of used Chinese characters, if any, among all the available versions of TT. In that case, in Table 1, we add another (less plausible in many cases) alternative in parenthesis that is given in his remarks, like " ". As seen in the table, the two Chinese characters "   " and "   ", which are similar to one another in form, are occasionally confused among the surviving versions of TT caused probably by a careless mistake in their transcription process. Keeping in mind the confusion, we checked the described dates for all the record using the words of not only "solar [lunar] eclipse ( )" and "lunar [solar] eclipse ( )" but also "solar eclipse ( )", and found that the described dates for some of these records correspond to "the full moon day (   )" or "someday around 15th day" in luni-solar calendar. Since such records can be judged to be lunar eclipse ones, they are included in Table 1.
3.2.  Contradiction in described date
We also see some contradiction in the described date in a few records. For example, record No.17 says " ", but "day (= 22nd day of the month)" does not correspond to "the full moon day (   )". We may conclude, however, that the record should describe a lunar eclipse because "  " is often used for a lunar eclipse and never used for a solar eclipse and also because a total lunar eclipse (lasting for 1h37m) was seen in Hanoi on "the full moon day (   )" of the month in almost the same way as described in the record. Next example is record No.24 saying " ", which gives contradictory date for a lunar eclipse. The contradiction can be resolved by assuming that only one character "  " has been added erroneously. On the other hand, in order to consider the record to be for a solar eclipse, we have to assume that two words "   "(or " ", "    ") and "   " have been erroneously " and "  ", respectively. It is also noticed that a lunar eclipse was seen in replaced by " Hanoi on " (=16th day of the month)" of the month while no solar occurred on the day of neither "   / (=1st day of the month)" nor "    (=the last day of the month)". Therefore, we may conclude that the record should also describe a lunar eclipse. ". Since "day Another example is record No.43 saying " (= 9th day of the month)" is favorable to neither lunar nor solar eclipse, it is very hard to infer, only from this description, which eclipse the record concerns. It is noted that record No.50 should be identical with record No.51 if we assume that one character " " has been erroneously replaced by " ", which is very similar to " " in form. As for record No.4, although the collated version by Chen Chingho (1984-86) does not make any remark on it, we find that some other versions of TT, including the Vietnamese edition published " not " in 1993 by the Institute of Hán Nôm in Hanoi, read " " for the two characters in the " in Chen Chingho's (1984-86) version. passage given as " We see here the confusion between "  " and "  " again. We adopt " " for the record so that the described date correspond to the date on which a lunar eclipse was seen in Hanoi.
3.3.  Possible one month/day shift
As mentioned in the previous section, the described date before AD1544 has been converted to the Julian Calendar date on the assumption that Vietnamese calendar is identical to Chinese one. Therefore we should keep in mind that there could exist one-month difference between Vietnamese and Chinese calendars due to a slight different way in inserting an intercalary lunar month in their calendars. Record No.1 says "A lunar eclipse was seen in April 15, 1169", which is exactly one month after the date when a lunar eclipse was seen in Hanoi. As mentioned above, this record might give a correct date in Vietnamese calendar, though we have no evidence for its correctness. Moreover, one-day shift could also arise for the period before AD1544 due to a slight difference between Vietnamese and Chinese calendars in arranging 29-day and 30-day months in a year. Apart from the calendar difference, the date of a historical lunar eclipse record is involved with the issue of day start time considered by people in those days, which will be discussed in later subsection.
Observed or predicted
3.4.
Some, probably many, of the lunar (and solar) eclipse records in historical sources of those days are believed to be those predicted by calendar calculation rather than actually observed. In the lunar eclipse records of in TT (Table 1), we find a few passages supporting this interpretation. For example, the records No.18 and No.28 says "There was an eclipse of the moon. It rained heavily. ( )" and "There was an eclipse of the moon. It was rainy, windy and dark. (The eclipse) was not seen. ( )", respectively. TT also contains an episode of the royal astrologist who was judged by the Imperial Court to have made a false lunar eclipse prediction. However, a slightly partial eclipse actually occurred at very low altitude in the sky from Hanoi in the almost predicted hour of the very predicted day (September 12, 1448) according to astronomical calculation.
3.5.  Reliability of records
By comparison with the results of astronomical calculation, among 55 lunar eclipse records in TT, we found that ten records (with symbol X in Table 1) yield an incorrect month for the lunar eclipse date. A few of those records may have been affected by the confusion of Chinese characters used there and also by a slight difference of Vietnamese calendar from Chinese one, as discussed above. Anyway, those 10 records should be regarded as doubtful. Besides them, we also found two records (with symbol in the table) giving a correct date, on which, however, no eclipse was observed in Hanoi: record No.5 and No.10 correspond to a penumbral eclipse and a day-time lunar eclipse, respectively. Although these two records are also regarded as doubtful, they may have remained rather free from the effect such confusion of characters as described above since 15th century because the described date is correct. If so, we may have some idea of the accuracy of Vietnamese calendar calculation in 15th century by analyzing these two records as well as other records in the same century. It may be noticed that the above mentioned episode of the astrologist also belongs to 15th century. Finally, we find that the remaining 43 records (with symbols in the table) are reasonable, and which correspond to 78% of all the lunar eclipse records in TT,
3.6.  Day start time
It is important to learn the day start time (i.e., the time indicating the boundary of a day) considered by people in old days because, without such knowledge, we cannot correctly specify the date for an event occurred at sometime in the night. For example, it is reported that medieval Japanese people considered the day start time to be around 3:00 am (e.g., Saito 1995). The dates described in lunar eclipse records of TT would give us some idea of the day start time considered by Vietnamese people in those days. Using all the records (32 records) for that we can specify their described date, we investigated the start and the end time of the lunar eclipse as well as the date. We find that, among the 32 lunar eclipses, 12 of them (group A) end before midnight while 14 of them (group B) start after midnight and that 6 of them (group C) runs over midnight. The results of our investigation are as follows: (1) All the twelve records of group A have given the same date as the daytime. (2) Nine of the fourteen records of group B have yielded the date of the next day while five have provided the same date as the daytime. (3) Five of the six records of group C have given the same date as the daytime while one has yielded the date of the next day. From these results, we can say that Vietnamese people did not consider a date as remaining unchanged all the night through. A more detailed study will be reported elsewhere.