Historical Archive of Sunspot Observations (HASO)
Welcome to HASO website.
Sunspot number series is the longest record of directly observed solar activity based on the compilation of historic and modern sunspot counts made with small and medium size telescopes since 1610 (covering more than 400 years). Sunspot number series have an important role in several branches of the modern science as astrophysics, solar physics and geosciences.
Successive great compilations by Wolf in the 19th century and Hoyt and Schatten in the 20th century have highlighted the importance of the recovery of the historical record to reconstruct the best sunspot number series.
The objective of HASO is to collect and preserve all documents in any format (original, photocopy, photography, microfilm, digital copy, etc.) with sunspot observations that can be used to calculate the sunspot number in the historical period or related documents.