Solar Observations by Angelo Secchi: An Analysis of His Diary and Annotations in His Sunspot Drawings

Abstract Angelo Secchi, a pioneer in solar physics, provided an extensive and regular record of sunspot observations spanning the period 1858–1878. This study analyzes the handwritten records by Secchi and collaborators in the form of drawings and an observation diary. We focus on the systematic annotations of group and spot counts, and sunspot area measurements. We present a machine-readable dataset that provides valuable information on historical solar activity, adding new data to limited series such as sunspot area measurements. This work compares Secchi’s sunspot records with modern solar activity series, evaluates the consistency of his records, and highlights their importance in extending long-term solar indices.