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 as regards the group count. Additionally, Schwabe was able to observe smaller groups than Bond.
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Acknowledgements
This research was supported by the Economy and Infrastructure Counselling of the Junta of Extremadura through project IB16127 and grants GR18081 (cofinanced by the European Regional Development Fund) and by the Ministerio de Economía y Competitividad of the Spanish Government (CGL2017-87917-P). The authors have benefited from the participation in the ISSI workshops led by M.J. Owens and F. Clette on the calibration of the sunspot number.
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All authors contributed to the study conception and design. The first draft of the manuscript was written by V.M.S. Carrasco and all authors read and approved the final manuscript.
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Carrasco, V.M.S., Gallego, M.C., Arlt, R. et al. Revisiting the Amplitude of Solar Cycle 9: The Case of Sunspot Observations by W.C. Bond. Sol Phys 295, 127 (2020). https://doi.org/10.1007/s11207-020-01696-y
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DOI: https://doi.org/10.1007/s11207-020-01696-y