Reconstruction of the Sunspot Number Series : Gathering Data

The International Sunspot Number (SN) is among the longest continuous records of solar activity, thus its accuracy is of the utmost importance for space climate studies. In that context, resolving the remaining scale changes and epoch-to-epoch inconsistencies requires a complete reconstruction of SN, i.e. SN V3.0. The success of this community wide effort depends first and foremost on a strengthened historical data foundation: locating dispersed sources, digitizing them, and preserving provenance-rich metadata so that calibration uncertainties can be linked to dataset quality, rather than only global statistical adjustments.
Within WDC–SILSO and the FARSuN project, major recovery campaigns have expanded and consolidated the observational base. Wolf’s Journals, the Mittheilungen (1610–1944) have been digitized into machine-readable form and integrated with ongoing metadata harmonization and cross-checks. The Zürich observation tables (1945–1979)—the backbone of modern daily production—are being completed through systematic digitization and observer metadata encoding. Complementary early-19th-century sources critical to the 1810s–1840s interval (e.g., Gruithuisen manuscripts, C. H. Adams drawings, and the Stark material) are being collected and extracted using dual-mode transcription (HTR/OCR plus manual verification) with quality control (alias reconciliation, calendar handling, NG/NS consistency tests, and two-person checks for fragile series).

By enlarging overlaps among observers and standardizing heterogeneous formats into a sustainable FARSuN historical sunspot database, these data-gathering efforts enable robust scale-transfer analyses and uncertainty-quantified products, providing the essential foundation for a defensible SN V3.0 series.