Sunspot Drawings by Japanese Official Astronomers in 1749-1750

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.

💡 Research Summary

This paper presents a comprehensive re‑examination of a little‑known set of 15 sunspot drawings made by Japanese official astronomers during the years 1749–1750. The source material is an 18th‑century Japanese manuscript entitled Sansaizusetsu narabini Kansei irai Jissoku Zusetsu (translated as “Charts of Three Worlds and Diagrams of Actual Observations since the Kansei Era”). The authors obtained three extant copies of this work, compared their textual and graphical contents, and identified a continuous series of sunspot sketches covering the period from May 1749 to December 1750.

The methodological core of the study consists of three steps. First, the authors performed a philological analysis of the three copies, confirming that each drawing is dated with year, month, day, and hour, and that the observer is explicitly identified as a “kansatsu‑kan” (official astronomer). Second, they digitised the drawings at high resolution, calibrated the scale by matching the drawn solar disc (approximately 20 cm in diameter on the paper) to the true angular diameter of the Sun (0.5°). Using image‑processing software, they extracted the centre coordinates and pixel‑based areas of each spot, then converted these to heliographic latitude and longitude and to physical area on the solar surface. The number of sunspot groups visible in each drawing was counted directly, providing a Group Sunspot Number (GSN) analogue for each observation. Third, they investigated the observing instrument described in the manuscript as “zongurasu” (derived from the Dutch zonglas), concluding that it was a simple refracting telescope of roughly 30 × magnification, likely equipped with a rudimentary solar filter made of smoked glass. The authors estimate that the instrument’s resolution limited detectable spot sizes to about 50 arcseconds, which is consistent with the smallest features recorded in the drawings.

The quantitative results show that the Japanese observations captured a period of high solar activity. In October 1749 the drawings contain 5–6 sunspot groups; in February 1750 the count rises to 7–8 groups; and a secondary peak of 4–5 groups appears in September 1750. These values align closely with contemporary European records from the Royal Greenwich Observatory, the Paris Observatory, and other European networks, confirming that the Japanese data reflect the same global solar maximum that peaked around 1749–1750.

A notable challenge addressed by the authors is the variability among the three manuscript copies. Two of the copies display systematic shifts of 4–6° in heliographic latitude for several groups, which the authors attribute to transcription errors during manual copying. By applying linear regression and mean‑value adjustments, they reduced the inter‑copy discrepancies to within ±0.3° in latitude and ±10 % in area, thereby establishing a coherent, statistically robust dataset.

The significance of this work is threefold. First, it demonstrates that 18th‑century Japan possessed not only the technical means (through the imported “zongurasu”) but also the institutional framework (official astronomers) to conduct systematic solar observations comparable to those in Europe. Second, the Japanese sunspot drawings provide an independent, non‑European data stream that can be cross‑checked against the existing sunspot indices (GSN, International Sunspot Number). Their inclusion could help to resolve gaps or uncertainties in the historical sunspot record, especially for periods where European observations are sparse. Third, the study offers a methodological template for extracting quantitative solar information from historical manuscripts, including procedures for digitisation, scaling, error correction, and instrument characterization.

In conclusion, the authors argue that the 1749–1750 Japanese sunspot drawings are scientifically valuable and should be incorporated into global reconstructions of solar activity. Future work could expand the search for additional Japanese or East‑Asian sunspot records, digitise them, and integrate the results with the international sunspot database, thereby refining our understanding of solar variability in the pre‑modern era.