Comparison of telescopic and naked-eye sunspots for the very small spots on February 15, 1900 and January 30, 1911

Abstract

In a recent discussion of the Maunder Minimum, two sunspot observations by Chinese court astronomers on February 15, 1900 and January 30, 1911 (±1 day) – presumably made with the unaided eye – were considered false detections because the spot areas of the largest spot on those days (±1 day) as recorded by the Royal Greenwich Observatory, would be too small for naked-eye detection, namely 11 and 13 millionths of a solar disk (msd), respectively (Usoskin et al. 2015). We revisit this issue here. First, we review theoretical and empirical considerations of the lower limit for the sunspot area detectable by the naked eye: under optimal conditions, very good observers can detect spots as small as ∼100 msd (and we present one example, where an observer reported a spot, when the largest spot on that day was only 65 msd, but being part of a longish group facilitating the detection). Then, we review all known sunspot observations on and around February 15, 1900 and January 30, 1911, including full-disk drawings. For February 15, 1900, Kalocsa observatory, Hungary, shows a feature close to the western limb with an area of 134 msd, but it is not clear whether it was a spot or faculae or pores (as spot, it could have been detectable even by naked-eye). The two spot groups detected in Kodaikanal, India, on January 31, 1911 and February 1 with 18.5 to 33.0 msd area would be too small for detection by the naked eye. However, the Chinese records for February 15, 1900 and January 30, 1911 do not even mention whether the observations were performed with a telescope or by the unaided eye. We conclude that there is no convincing evidence that these two – or even all – Chinese sunspot records are unreliable.