Revisiting the Amplitude of Solar Cycle 9: The Case of Sunspot Observations by W.C. Bond

TL;DR

This study revisits William Cranch Bond's 19th-century sunspot observations, correcting previous counts and providing a more accurate assessment of Solar Cycle 9's activity, with implications for historical solar activity records.

Contribution

The paper corrects Bond's sunspot group counts using original drawings, revises the highest daily and annual activity records, and compares Bond's data with Schwabe and Wolf observations.

Findings

Bond's sunspot counts were overestimated due to data misinterpretation.

Schmidt recorded the highest daily sunspot groups in Solar Cycle 9.

Bond's and Wolf's statistics are similar, but Schwabe observed smaller groups.

Abstract

William Cranch Bond, director of the Harvard College Observatory in mid-19th century, carried out detailed sunspot observations during the period 1847-1849. We highlight Bond was the observer with the highest daily number of sunspot groups observed in Solar Cycle 9 recording 18 groups on 26 December 1848 according to the current sunspot group database. However, we have detected significant mistakes in these counts due to the use of sunspot position tables instead of solar drawings. Therefore, we have revisited the sunspot observations made by Bond, establishing a new group counting. Our new counts of the sunspot groups from Bond's drawings indicate that solar activity was previously overestimated. Moreover, after this new counting, Bond would not be the astronomer who recorded the highest daily group number for Solar Cycle 9 but Schmidt with 16 groups on 14 February 1849. We have also indicated the new highest annual group numbers recorded by any observer for the period 1847-1849 in order to correct those values applied in the "brightest star" method, which is used as a rough indicator of the solar activity level. Furthermore, a comparison between Bond's sunspot records and the sunspot observations made by Schwabe and Wolf is shown. We conclude that the statistics of Wolf and Bond are similar regarding to the group count. Additionally, Schwabe was able to observe smaller groups than Bond.