Sunspot observations with telescopes in 18th century were carried out in Japan as well. One of these sunspot observations is recorded in an account called Sansaizusetsu narabini Kansei irai Jissoku Zusetsu (Charts of Three Worlds and Diagrams of Actual Observations since Kansei Era). We analyze manuscripts of this account to show in total 15 sunspot drawings in 1749-1750. These observations were carried out by contemporary official astronomers in Japan, with telescopes covered by zongurasus (< zonglas in Dutch, corresponding to "sunglass" in English). We count their group number of sunspots to locate them in long-term solar activity and show that their observations were situated around the solar maximum in 1749 or 1750. We also computed their locations and areas, while we have to admit the difference of variant manuscripts with one another. These observational records show the spread of sunspot observations not only in Europe but also in Japan and hence may contribute to crosscheck or possibly improve the known sunspot indices.
Solar activity is reviewed and measured by appearance of sunspots on the solar disc (Vaquero & Vázquez, 2009). It is in the early 17 th century that telescopic observations for sunspots started to offer scientific datasets from one of the longest-running experiments in human history (Owens, 2013). These sunspot observations were used to reconstruct past solar activity. R. Wolf and his successors in Zürich construct the Zürich number (Wolf number) to cover solar activity since 1700 (Waldmeier, 1961). Hoyt and Schatten (1998) include more observations to construct group sunspot number since 1610s.
Recent studies revisit the sunspot number (Clette et al., 2014;Svalgaard and Schatten, 2016;Vaquero et al., 2016;Willamo et al., 2017), based on recent discussions on original sunspot drawings and sunspot counting within scientific documents after telescopic observations since 1610s (e.g., Vaquero, 2007;Vaquero and Vázquez, 2009;Arlt, 2008Arlt, , 2009Arlt, , 2011;;Arlt, 2009;Arlt and Fröhlich, 2012;Pavai et al., 2016;Cliver and Keer, 2012;Diercke et al., 2014;Usoskin et al., 2015;Willis et al., 2013Willis et al., , 2016aWillis et al., , 2016b;;Arlt et al., 2016;Carrasco et al., 2016Carrasco et al., , 2018;;Hayakawa et al., 2018aHayakawa et al., , 2018b;;Svalgaard, 2017). Within these datasets, sunspot drawings are of greater value as they have information not only on sunspot number but also on their area, distribution, locations, configuration, and so forth (Vaquero, 2007;Vaquero and Vázquez, 2009).
While most of the early sunspot drawings down to the end of 19 th century were from Europe (Vaquero and Vázquez, 2009;Vaquero et al., 2016), recent findings of early sunspot drawings from non-European countries contribute to improve the reconstruction of past sunspot index (e.g., Domínguez-Castro et al., 2017;Denig and McVaugh, 2017). The Japanese archives may also contribute to this reconstruction by their contemporary sunspot drawings. Hoyt & Schatten (1998) seem partially aware of sunspot drawings by Kunitomo Ikkansai (國友一貫齋)1 during 1835-36 (Yamamoto, 1937;;Kubota & Suzuki, 2003). However, even before his sunspot observations, it is partially mentioned that we had some sunspot observations in Japan in 18 th century (Kanda, 1960;Watanabe, 1987). One of them is sunspot drawings in manuscripts of Sansaizusetsu narabini Kansei irai Jissoku Zusetsu which can be translated as Charts of Three Worlds and Diagrams of Actual Observations since Kansei Era (三際圖説 並寛政以來實測圖説, hereafter, SKJZ). In this article, we examine SKJZ to show their sunspot drawings and relevant records with their digitalization. We also count their group number of sunspots and compare them with contemporary solar activity.
SKJZ has two manuscripts with sunspot drawings (KS 2 : v.3, p.782). Both of them are preserved in Tohoku University at Sendai, Japan as shown below. We show their references with abbreviation, reference number, and hosting library. MS/K: MS 8-21318-1 in Kano Library of Tohoku University Library MS/O: MS 911-17799 in Okamoto Library of Tohoku University Library
We first introduce the characteristics of these manuscripts and show their sunspot drawings. We then analyze their text to estimate the observer and observational method. Then we count their group number of sunspots to locate them in the records to long-term solar activity. We also scaled the area of sunspots for their projected area and corrected area, before and after removing foreshortening effects. In this counting, we grouped sunspots according to the Zürich classification (e.g. Waldmeier, 1947;Kiepenheuer, 1953).
As described above, we have two manuscripts of SKJZ with sunspot drawings. Colophons of both manuscripts relate their author or compiler with Watanabe Masanami (渡部将南), although his name is not found in other contemporary documents. MS/K consists of 31 folios while MS/O consists of 27 folios. Their difference is found in additional folios of MS/K (ff.27b-31a) 3 for drawings of comet observations on 1811 September 15, 1819 July 17, 1824 January 02, and 1825 October 03. On contrary, we can find two graffiti in red letters in MS/O (ff.2a-2b).
These manuscripts start with Sansaizusetsu to explain three phases from the ground to the upper sky generating thunder in Japanese traditional understanding (ff.1a-4b) and cover astronomical and meteological observations such as comets, sunspots, and solar halos from 1758 to 1825. We estimate MS/K and MS/O compiled at least after 1825 and 1803 according to their last date of observations: comet drawing dated 1825 for MS/K and drawing of halo dated 1803 for MS/O. These manuscripts include considerable amount of earlier observations copied from preceding sources including Watanabe Masanami’s works.
2 Kokusho Soumokuroku: a union catalogue for early Japanese books. Note that we have two other variants entitled as Sansaizusetsu (Charts of Three Worlds) without Kansei irai Jissoku Zusetsu (Diagrams of Actual Observations since Kans
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