学会誌 要約紹介　通巻3　2021年1巻1号

Regarding the Oyu Stone Circles, which is a representative stone circle in Japan, the author pointed out the deficiencies in the rock layouts depicted in past reports. Then, the author presented a map of the rock layout, which is believed to be correct at this time, with the orientation of the sun as of 2000 BC.

The Oyu Stone Circles are said to have been built with the summer solstice in mind.

However, the line connecting the center of the Manza Stone Circles and the sundial stone arrangement deviates from the direction of sunset at the summer solstice, so the author asked this theory to reconsider.

It is said that one of the roles of the Oyu Stone Circles is an astronomical observation . The terrain in which the sun enters and exits as seen from the sundial stones in the ruins was identified by astronomical simulation, and it was verified whether it was a characteristic terrain. The results confirmed that the sun was clearly entering and exiting the intentional terrain. In addition, assuming that the colonnade at the ruins is a star observation device, we verified which star it points to by astronomical simulation. As a result, it became clear that the appearance of the dawn of Sirius might have predicted the harvest time of chestnuts, which is important for the Jomon people. From this, it can be said that the archaeological site Oyu Stone Circles is an astronomical observation site.

The Kanayama Megaliths Research Center is currently conducting an extension of the Nakatsugawa City megalith group survey that started in 2018. It extends the survey over a range of about 48 km to Seki City. So far, at least 14 megaliths have been confirmed on the east-west line at latitude 35 degrees 31 minutes 40 seconds north. The locations include Nakatsugawa City, Ena City, Yaotsu Town, Kawabe Town, Minokamo City, and Seki City. Among them, there are seven megaliths within a range of about 2.5 km from Seki City Bouji to Nishikamino, and the megaliths are lined up at intervals that allow them to be visible to each other. In these megalithic groups, there are mainly observations of the rising or setting sun in the spring and autumn equinoxes, and megalithic structures that can observe the North Star.Estimating the age from the observation of the North Star, one theory is based on the time when Thuban in Draco was shining as the North Star; it was about 4500 years ago due to the influence of the precession of the Earth. In other words, it can be thought of as a megalithic observatory dating back to the Jomon period, similar to the Kanayama megaliths. Also, from the archaeological background, the east-west line of about 48 km runs through the area where Jomon archaeological sites are concentrated, and it overlaps with the east-west line of the old mountain road.   It can be considered that these megalithic groups were deeply connected to the people living with the sun during the Jomon period, and that a megalithic network was constructed through the communication of people in various places.

In the previous paper, it was revealed that by restoring Suzaku-oji Street of Fujiwara-kyo, two Imperial capital cities with different deflection angles of the central axis, Fujiwara-kyo created in the period of Empress Jito and Aramashi-kyo created in the period of Emperor Monmu, overlapped in a two-layered structure, and Daigokuden in Fujiwara-kyo was transformed into Suzaku-mon Gate in Aramashi-kyo. In this paper, by restoring Shijo-oji Street, I clarified that Nishi-gobo Street was at the west end of Fujiwara-kyo. As a result, a Jobo standard was proposed for each of the two overlapping Kyo, and both Fujiwara-kyo and Aramashi-kyo were restored to nine Bo-districts through north to south, and ten Bo-districts through east to west. A capital city very similar to this is Kin-kyo in Silla. To replace Daigokuden of Fujiwara-kyo with Suzaku-mon Gate of Aramashi-kyo, Aramashi-kyo had a double-sized-Jobo standard compared to Fujiwara-kyo. On Shijo-oji Street, as on Suzaku-oji Street, the phenomenon of Jobo's leftward rotation can be detected, and it was possible to confirm the time-dependent leftward rotation of the Asuka type 'Celestial Central Axis'. Most of the gutter remnants of precedence Jobo inside of the Fujiwara Palace were dug during the period from 704 to 705, when Fujiwara-kyo was expanded to Aramashi-kyo.

The transfer of the Imperial palace from Fujiwara Palace to Aramashi Palace took place in April 705, but the calamity never stopped, and it seems that the Yamato Court had already admitted the failure of the relocation of the palace in June of the year, then started to consider transferring the capital.

The two main axes, the east-west and the north-south direction, coexist in Chang'an Castle in the Former Han, and these main axes are set based on the King Wen bagua diagram. The major axis in the east-west direction is the Zhen-Dui line in the diagram, which depicts the Celestial Central Axis connecting the vernal equinox (east) and the autumnal equinox (west) at the sunrise on the vernal equinox on the ground.

When Gaozu Liu Bang founded Han, it is said that the auspicious omen of five planets gathered at the Well mansion appeared in October of the first year of Emperor Gao's reign. The celestial phenomena at that time were pictured on the ground, and they were the Tianqi Shrine and the five planet-stylobates remaining on the Tianjing'an village on the Loess Plateau. The five planet-stylobates were arranged based on the Silk Texts bagua. The major axis in the north-south direction is a straight line with a total length of 77 km starting at the Mercury stylobate and ending at the Meridian Valley south end in Nanshan, passing through the South Gate of Changling and the South Gate of Chang'an Castle along the way. The northern end of the main axis is the Mercury stylobate, which proves that the Virtue of the early Former Han dynasty was the Water phase of the Wuxing. This major axis is the Kan-Li line in the King Wen bagua, which depicts the Celestial Central Axis connecting the Wall mansion, the Tenno Taitei（天皇大帝）, the Zenith and the determinative star of the Chariot mansion that appeared just before the sunrise in October of the first year of Emperor Gao's reign on the ground. The Celestial Central Axis was accompanied by the appearance of the Celestial Circle (the Milky Way which looks like a circle), which is why Han used 'Han（漢）' as the name of the dynasty. In addition, the shape of the north wall of the castle was modeled after the shape of the Big Dipper at that time, and the shape of the south wall of the castle was modeled after the Milk Dipper（i.e. the Dipper mansion）, which was the place where the sun was moving south in the daytime. This is why Chang’an Castle was called the Dipper Castle（斗城）.

The Celestial Central Axis in the east-west direction does not rotate with time, but the Celestial Central Axis in the north-south direction rotates to the left with time due to the Axial precession effect. The deflection angle of the major axis in the north-south direction at Chang’an Castle is +0.0914 degrees, which corresponds to the angle of the Celestial Central Axis in the north-south direction in 177 B.C. It is considered that the north-south axis was actually set in 201 B.C., which means that Han used the Celestial Central Axis 24 years later to set the north-south axis of the Castle. Therefore, it is considered that the procedure of the Scheduled Year was already introduced in the Former Han period. 

  上記の課題設定に沿って、2019年４月には科学研究費の助成を受け「考古天文学会議」を立ち上げた。きっかけは拙著『古墳の方位と太陽』と後藤明氏の著作『天文の考古学』が共に2017年５月に同成社から刊行されたことであった。後藤氏は天文学の知見を援用した文化人類学・考古学の構築を目指していたので、両著の刊行を機に意見交換をする機会に恵まれ、相互の基本認識で一致した。後藤氏を介して国立天文台に所属する関口和寛氏や高田裕行氏など天文学関係者との連携が実現することになった。

　さらに『歴史のなかの大地動乱−奈良・平安の地震と天皇−』（岩波新書、2012年刊）の著者でもある保立道久氏と私は、景観史学の共同研究を進めてきたこともあり、今回の課題設定もその延長線上にあった。ただちに天文現象への関心を抱く文献史学者への打診がおこなわれ、現在の研究体制が構築されたのである。私の目からみても異色のメンバー構成だと思う。

　ちなみに「考古天文学」は後藤氏の提案に沿った命名である。以前に斉藤国治が提唱した「古天文学」とも響き合う。なお、貴学会が先行して設立されたこともあり、新たな名称が求められたという背景もある。

　なお日本考古学界の現状をみれば、過去の天体景観に学問的な関心が向けられる情勢だとは言いがたい。というより、まだ拒絶的な態度が堅持される段階だとみるべきであろう。戦後の日本考古学や古代史学がたどった研究動向と深く連動しているからである。

　考古学については、皇国史観からの脱却すなわち記紀神話との決別が主要な命題となり、虚構を排して科学的な研究法を構築する必要があった。そのさい重視されたのはマルクス主義的な史的唯物論であった。社会は生産や経済活動（これを下部構造と呼ぶ）がベースとなり、祭祀や政治（これを上部構造と呼ぶ）はそれに規定され、社会構造は生産様式として把握される、といった理論的な枠組みである。この理論に依拠すると、天体景観への認知や信仰は、観念領域に属する上部構造の問題とみなされる。だから重視すべき下部構造つまり生産や経済とは切り離され、棚上げされたのである。上部構造のなかで最重要視されたのも支配体制の問題であった。

　ただし、そのような模索期間も終了しつつあり、現在は新たな局面への移行期にさしかかっている。景観史への認知度が高まってきたからであり、観念領域で生じた変革が社会に与えた影響の強さにも学界全体が気づき始めたからである。

　さらにGPSの技術的な発展は、天体景観を含む全体景観を的確に再現することを可能にし、新たな研究環境を用意してくれている。この環境を活かさない手はない。考古天文学会議は、このような現状認識に立脚するものである。

　現在はもう1件の科学研究費助成金を原資に、関口氏の主導のもと、science Nodeの岩城邦典氏が技術部門を担当するかたちで、各地・各時代の遺跡・遺構からみた過去の全体景観を再現するシミュレーション・システム〈arc Astro VR〉が構築されつつある。本システムはほぼ完成の域に達しており、日本語と英語のマニュアルを添えて2022年度上半期には国立天文台から無料で公開される予定である。

　吉野ヶ里歴史公園を会場に開催された第４回考古天文学会議では、そのお披露目を兼ねたデモンストレーション映像を開示した。次回の世界考古学会議では後藤氏がプレゼンをおこなう予定である。

　では2019年以降に解明された研究成果の一端を紹介する。2017年の拙著では太陽の運行と北天の周回星（現在の北極星「鉤陳」や北斗七星の運行）を問題にしたが、現在は月の問題に重点を移している。そこで地上からみた月の運行の特徴や太陽の運行との関係を解説することから始めたい。また研究の経緯についても記述する。