Abstract
The Maunder Minimum is the period between 1645 and 1715. Its main characteristic is abnormally low and prolonged solar activity. However, some authors have doubted the low level of solar activity during that period by questioning the accuracy and objectivity of the observers. This work presents a particular case of a sunspot observed during the Maunder Minimum with an unusual shape of its umbra and penumbra: a hexagon. This sunspot was observed by Cassini in November 1676, just at the core of the Maunder Minimum. This historical observation is compared with a twin case that occurred recently in May 2016. The conclusion reached is that Cassini’s record is another example of the good quality of the observations that were made during the Maunder Minimum, showing the meticulousness of the astronomers of that epoch. This sunspot observation made by Cassini does not support the conclusions of Zolotova and Ponyavin (Astrophys. J. 800, 42, 2015) that professional astronomers in the seventeenth century only registered round sunspots. Finally, a discussion is given of the importance of this kind of unusual sunspot record for a better assessment of the true level of solar activity in the Maunder Minimum.
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Acknowledgements
All the historical materials used in this work were consulted at the Historical Archive and Library of the Real Instituto y Observatorio de la Armada, San Fernando (Cádiz), Spain. This research was supported by the Economy and Infrastructure Counselling of the Junta of Extremadura through project IB16127 and grant GR15137 (co-financed by the European Regional Development Fund) and by the Ministerio de Economía y Competitividad of the Spanish Government (AYA2014-57556-P and CGL2017-87917-P).
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Carrasco, V.M.S., Vaquero, J.M. & Gallego, M.C. Could a Hexagonal Sunspot Have Been Observed During the Maunder Minimum?. Sol Phys 293, 51 (2018). https://doi.org/10.1007/s11207-018-1270-0
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DOI: https://doi.org/10.1007/s11207-018-1270-0