On the Uncertainties of the Sunspot Area Measurements

Abstract This study examines several sources of uncertainty affecting sunspot area measurements, with particular emphasis on projection effects caused by the curvature of the Sun. Using simple geometric models, we derived analytical and approximate corrections for sunspots with circular and elliptical shapes. Although projection uncertainties are generally small, they become increasingly relevant for large sunspots. Among the various sources of uncertainty, thresholding and perspective effects emerged as the most significant. In contrast, the impact of the image resolution was found to be relatively minor. The corrections proposed here provide a straightforward way to refine area estimates and can be applied to both modern digital data sets and historical sunspot records. This contributes to a more consistent and homogeneous reconstruction of long-term solar activity trends.