Revisiting the Amplitude of Solar Cycle 9: The Case of Sunspot Observations by W.C. Bond
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Revisiting the Amplitude of Solar Cycle 9: The Case of Sunspot Observations by W.C. Bond V.M.S. Carrasco1,2 J.M. Vaquero2,4
· M.C. Gallego1,2
· R. Arlt3 ·
Received: 19 April 2020 / Accepted: 28 August 2020 / Published online: 22 September 2020 © Springer Nature B.V. 2020
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. Keywords Sunspot number · Solar activity · Solar Cycle 9
1. Introduction The sunspot record carried out by astronomers since December 1610, when Harriot recorded the first sunspot observation by telescope, is considered as the longest ongoing system-
B V.M.S. Carrasco
[email protected]
1
Departamento de Física, Universidad de Extremadura, 06006 Badajoz, Spain
2
Instituto Universitario de Investigación del Agua, Cambio Climático y Sostenibilidad (IACYS), Universidad de Extremadura, 06006 Badajoz, Spain
3
Leibniz Institute for Astrophysics Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
4
Departamento de Física, Universidad de Extremadura, 06800 Mérida, Spain
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atic direct observation set in the world (Owens, 2013; Arlt and Vaquero, 2020). The sunspot number indices are calculated from the number of sunspots recorded during the telescopic era (Vaquero, 2007; Clette et al., 2014; Usoskin, 2017). Several authors have detected problems in the methodology used to calculate these indices (Svalgaard, 2010; Cliver and Ling, 2016; Lockwood, Owens, and Barnard, 2016). For that reason, new sunspot number series have been published to solve it (Lockwood et al., 2016; Clette and Lefèvre, 2016; Svalgaard and Schatten, 2016; Usoskin et al., 2016; Chatzistergos et al., 2017; Willamo, Usoskin, and Kovaltsov, 2
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