The sunspot observations by Samuel Heinrich Schwabe

A long time-series of sunspot observations is preserved from Samuel Heinrich Schwabe who made notes and drawings of sunspots from 1825-1867. Schwabe’s observing records are preserved in the manuscript archives of the Royal Astronomical Society, London. The drawings have now been digitized for future measurements of sunspot positions and sizes. The present work gives an inventory and evaluation of the images obtained from the log books of Schwabe. The total number of full-disk drawings of the sun with spots is 8486, the number of additional verbal reports on sunspots is 3699. There are also 31 reports about possible aurorae.

💡 Research Summary

This paper presents a comprehensive digitization and analysis of the sunspot observations made by Samuel Heinrich Schwabe between 1825 and 1867. Schwabe’s original logbooks, consisting of 39 bound volumes held by the Royal Astronomical Society in London, contain a total of 8,486 full‑disk sun‑spot drawings and 3,699 verbal reports without accompanying sketches. The authors photographed each page with a Canon EOS 5D equipped with a low‑distortion 50 mm f/2.8 lens, achieving a spatial resolution of roughly 0.07–0.08 mm per pixel (29 12 × 437 8 pixels per page). Raw 12‑bit files were processed in RawTherapee to preserve the subtle pencil marks while minimizing paper‑defect artifacts. Individual day images were extracted and named using the format “YYYYMMDDHHMM.JPG”, with “0000” used when Schwabe gave no explicit time.

The digitized dataset is categorized into three groups: (1) drawings with a coordinate grid (8,486 entries), (2) drawings lacking a coordinate grid (1,168 entries), and (3) verbal reports without any drawing (3,699 entries). The presence or absence of a coordinate system is crucial for later conversion of the plotted positions into heliographic latitude and longitude; the 1,168 grid‑less drawings will require additional orientation calibration. The verbal reports, while valuable for tracking the appearance and disappearance of spots, lack precise timestamps, which complicates automated time‑series construction.

Schwabe’s observing routine was conducted primarily from his rooftop tower in Dessau (λ = 12° 14′ 32″ E, φ = 51° 50′ 19″ N). He used several Keplerian refractors throughout his 43‑year career. Early instruments included a 2.5‑foot Ramsden telescope (unknown aperture) and a 1.5‑foot Thomas Harris instrument. In 1826 he acquired a 3.5‑foot Fraunhofer refractor, which became the main tool for the majority of his drawings from 1826 onward. A larger 6‑foot Fraunhofer telescope (12.2 cm aperture) obtained from Wilhelm Gotthelf Lohrmann in 1829 was also employed. All telescopes were used with various colored absorption glasses (blue, violet, pale‑blue) to reduce solar glare. The authors list the focal lengths, manufacturers, and first‑use dates for each instrument, providing essential metadata for assessing the optical resolution and contrast of the historic drawings.

The paper also quantifies the temporal distribution of observations. Monthly totals (Fig. 4) reveal a clear seasonal pattern: fewer observations in winter months and a concentration of daily reports during the years 1840–1845. Schwabe’s meticulous counting of spotless days enabled him to announce the 11‑year solar cycle in 1844, a discovery later confirmed by Wolf and others. The dataset includes 1,168 spotless days, offering a valuable cross‑check on modern sunspot number reconstructions.

Beyond sunspots, Schwabe recorded faculae (referred to as “Lichtwolken”), limb darkening (noted in 1841, 1846, 1847, and 1851), and granulation (“the marbled surface of the Sun”). These qualitative observations can be compared with modern high‑resolution solar imagery to investigate long‑term changes in solar photospheric contrast. The authors note that faculae are sometimes difficult to distinguish from sunspots in the pencil sketches; however, the accompanying verbal notes and the absence of group numbers for faculae provide useful clues.

A particularly interesting ancillary dataset consists of 31 reports of possible aurorae. These events cluster near solar maxima, suggesting a correlation that could be explored in historical space‑weather studies. The paper also lists Schwabe’s observations of comets, asteroids, the discovery of Neptune (September 1866), and attempts to locate a hypothetical intra‑Mercurial planet predicted by Le Verrier in 1860.

The authors argue that when combined with the Greenwich photographic record (starting 1874) and the earlier observations of Gustav Spörer and Richard Carrington, Schwabe’s data enable the construction of a continuous, high‑cadence sunspot position and area series from 1825 to the present. Such a series would be invaluable for testing solar dynamo models, improving reconstructions of solar irradiance, and refining long‑term climate‑solar forcing relationships. By making the digitized images and metadata publicly available, the authors provide a foundation for the global community to perform precise heliographic measurements, assess the quality of 19th‑century telescopic optics, and integrate these historic observations into modern solar physics databases.