A New Look at a Centuries-Old Solar Eclipse

Humans have recorded solar eclipses for thousands of years. Recently, researchers translated and analyzed solar eclipse observations from 18th-century Sweden to determine the Sun’s radius at that time and study the first confirmed record of solar prominences.

Looking Back Nearly 300 Years

On 13 May 1733, a solar eclipse darkened skies across a swath of modern-day Canada, Greenland, Iceland, and Scandinavia. Among the eclipse’s careful observers was Swedish astronomer and mathematician Birger Wassenius, who dutifully sketched the eclipse and the massive loops of plasma and magnetic fields — solar prominences — that bulged out from behind the Moon’s obscuring disk.

Wassenius penned a short account of his observations in Latin for Philosophical Transactions of the Royal Society (the longest-running scientific journal in the world!). His drawings of the event, which include the first robust record of solar prominences, were never officially published, but were housed in the Royal Academy Archives of Sweden. Now, researchers have translated Wassenius’s full report from Swedish to English and analyzed his sketches, helping to fill an important gap in the historical record of solar activity.

Drawing from Observations

Hisashi Hayakawa (Nagoya University; Rutherford Appleton Laboratory) and collaborators translated Wassenius’s notes, which describe how he viewed the eclipsed Sun through a 21-foot-long telescope during just over two minutes of totality. From Wassenius’s measurements, Hayakawa and coauthors calculated the solar radius to be 696,250 ± 170 km; this is larger than the modern accepted radius, but it’s in line with what has been calculated from solar eclipse measurements two decades earlier, in 1715.

Wassenius also made several sketches of prominences hovering above the edge of the Sun. While assessing these drawings, Hayakawa and coauthors had to contend with the imperfections of these hand-recorded features. With a narrow field of view that prevented him from seeing the entire disk at once, and with his time limited by the brief eclipse, Wassenius scribbled in two of the three prominences after the fact, from memory. After assigning reasonable uncertainties to the positions of these prominences, the team determined that at least one of these features was located at a high latitude.

Prominent Features

High-latitude prominences like the one (or more) seen by Wassenius are most common during what’s known as the “polar rush,” which occurs as solar activity rises toward a maximum. If these prominences are related to the polar rush, that would mean that in 1733, the Sun was not in an activity minimum as previously thought. However, it’s also possible for prominences to pop up at high latitudes during other times in the solar cycle; the authors estimated the likelihood of this happening at 20%.

This work shows the value of historical observations in determining the Sun’s past activity cycles. The 1733 eclipse observation by Wassenius adds a fascinating data point to the long history of solar eclipse observations, helping to illuminate a time when sunspot data were scarce.

Citation

“Analyses on Wassenius’s Report for Total Solar Eclipse in 1733: Quantifications of the Solar Radius and the Earliest Reported Prominences,” Hisashi Hayakawa et al 2026 ApJ 1007 189. doi:10.3847/1538-4357/ae6451