Rediscovery of 23 Historical Records of Naked-Eye Sunspot Observations in AD 1618

Abstract

The year 1618 was once regarded as a minimum of the first observed solar cycle or even the beginning of the extended Maunder minimum. However, new results from the annual dataset of radiocarbon (Usoskin et al., Astron. Astrophys. 649, A141, 2021) shows that 1620 was the solar minimum, instead of the year 1618. We revisited the sunspot activity in 1618 from historical records of naked-eye sunspot observations (HRNSOs) in China, as daily telescopic observations were found on only 28 days in 1618, and they are far from sufficient to resolve the difference. We rediscovered 23 HRNSOs from 1618 by a search of more than 800 historical books, with 15 HRNSOs identified as independent observations. From the sunspot records rediscovered here, the Chinese had seen several large sunspots in 1618. On 20 and 21 June 1618, the vapor-like sunspots were so large that even ordinary people could see them and thought that the Sun was abnormal. On 22 June 1618, at least three Chinese observers reported a huge group as a diffusive, round, and vapor-like object with an impressive size. The 23 HRNSOs rediscovered here provide valuable observations to determine the sunspot activity in the year 1618. Our result confirms the one from the annual dataset of radiocarbon, and shows that the year 1618 was quite active.