Revisiting Kunitomo’s Sunspot Drawings During 1835 – 1836 in Japan

Abstract

We revisit the sunspot drawings made by the Japanese astronomer Kunitomo Toubei during 1835 – 1836 and recount the sunspot group number for each image. There are two series of drawings, preliminary ($P$, containing 17 days with observations) and summary ($S$, covering 156 days with observations), all made using brush and ink. $S$ is a compilation of drawings for the period from February 1835, to March 1836. Presently, the $P$ drawings are available only for one month, September 1835; those of other periods have presumably been lost. Another drawing ($I$) lets us recover the raw group count (RGC) for 25 September 1836, on which the RGC has not been registered in the existing catalogs. We also revise the RGCs from $P$ and $S$ using the Zürich classification and determine that Kunitomo’s results tend to yield smaller RGCs than those of other contemporary observers. In addition, we find that Kunitomo’s RGCs and spot areas have a correlation (0.71) that is not very different from the contemporary observer Schwabe (0.82). Although Kunitomo’s spot areas are much larger than those determined by Schwabe due to skill and instrument limitations, Kunitomo at least captured the growing trend of the spot activity in the early phase of Solar Cycle 8. We also determine the solar rotation axis to estimate the accurate position (latitude and longitude) of the sunspot groups in Kunitomo’s drawings.