Some studies have shown that our knowledge of the solar activity in the years 1640, 1652, and 1741 can be improved. In this contribution, we revise the annual group sunspot numbers for these years from original observations. For the years 1640, 1652, and 1741, we have obtained the corrected values 15.2, 1.8, and 27.3, respectively (instead of the original values 15.0, 4.0, and 57.7).
Sunspot number Group sunspot number Solar activity
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The authors thank the Harvard University Library for providing a copy of the observation of Winthrop. J.M. Vaquero has benefited from the impetus and participation in the Sunspot Number Workshops (http://ssnworkshop.wikia.com/wiki/Home). Support from the Junta de Extremadura (Research Group Grant No. GR10131) and Ministerio de Economía y Competitividad of the Spanish Government (AYA2011-25945) is gratefully acknowledged. Ricardo M. Trigo was supported by the Portuguese Science Foundation (FCT) through project QSECA (PTDC/AAG-GLO/4155/2012).
IPCC: 2007, Climate Change 2007: the Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, University Press, Cambridge.