Analyses on Wassenius' Report for Total Solar Eclipse in 1733: Quantifications of the Solar Radius and the Earliest Reported Prominences

This study analyzes a 1733 manuscript by Wassenius to quantify the solar radius and document early reports of high-latitude prominences, providing insights into the solar cycle and the potential timing of the solar minimum during that period.

The Gist

The Solar Time Machine: Unlocking a 300-Year-Old Secret

Imagine you found a dusty, handwritten diary from a traveler in the year 1733. This diary doesn't just talk about the weather; it contains the first-ever eyewitness account of "solar prominences"—giant, glowing loops of fire leaping off the surface of the Sun.

That is essentially what scientists have just done. They took an unpublished, 293-year-old manuscript by a Swedish mathematician named Birger Wassenius and used modern supercomputers to turn his sketches into high-tech solar data.

Here is the breakdown of their "time travel" discovery:

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1. The "Magnifying Glass" of an Eclipse

To understand the Sun, you usually need massive satellites. But a Total Solar Eclipse is like nature providing a free, high-definition magnifying glass. For a few brief minutes, the Moon slides perfectly in front of the Sun, acting like a cosmic shutter that blocks the blinding glare, allowing us to see the Sun’s "atmosphere" (the corona) and the fiery structures (prominences) dancing on its edges.

Wassenius used a 21-foot telescope—which, in 1733, was like trying to look at a distant lighthouse through a long, narrow straw. He couldn't see the whole picture, but he saw enough to change science forever.

2. Measuring the Sun with a Stopwatch

The researchers did something clever: they used Wassenius’s notes on time. He recorded exactly how many seconds the darkness lasted (128 seconds).

The Analogy: Imagine trying to figure out the size of a giant beach ball by watching how long it takes a tiny ant to walk across its shadow. By matching Wassenius’s "stopwatch" timing with modern maps of the Moon’s bumpy surface (the craters and valleys), the scientists were able to calculate the Sun's size back then.

The Discovery: They found that the Sun in 1733 was slightly "puffier" than it is today. It’s as if the Sun breathes, expanding and contracting very slightly over centuries.

3. The Mystery of the "High-Latitude" Fireballs

This is the most exciting part. Wassenius sketched three bright spots (prominences) appearing at the very top and bottom of the Sun.

The Analogy: Think of the Sun like a spinning disco ball. Usually, the "action" (sunspots and solar activity) happens around the "equator" (the middle). If you suddenly see bright lights flashing at the very North and South Poles, it tells you something big is happening with the Sun's internal engine.

According to the old records, 1733 was supposed to be a "Solar Minimum"—a period when the Sun is quiet and sleepy, like a calm ocean. But Wassenius’s "fireballs" at the poles suggest the Sun wasn't actually sleeping; it was actually in the middle of a transition, perhaps waking up from a nap earlier than we thought.

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Why does this matter?

You might ask, "Why care about a guy with a telescope from 300 years ago?"

Because the Sun is the engine of our solar system. It affects our satellites, our power grids, and our space weather. To predict how the Sun will behave in the future, we need to understand its long-term "mood swings."

By translating Wassenius’s old Swedish sketches into modern math, these scientists have added a vital new chapter to the Sun's biography, helping us understand the rhythm of the solar cycles that govern our cosmic neighborhood.

Technical Summary

Technical Summary: Analyses on Wassenius’ Report for Total Solar Eclipse in 1733

Title: Analyses on Wassenius’ Report for Total Solar Eclipse in 1733: Quantifications of the Solar Radius and the Earliest Reported Prominences
Authors: Hayakawa et al. (2026)
Journal: The Astrophysical Journal

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1. Problem Statement

The study addresses two primary gaps in solar physics and historical astronomy:

• Solar Radius Variability: There is ongoing scientific debate regarding the long-term variability of the solar radius ($R_\odot$) over centennial timescales. Historical data from the 18th century are sparse, making it difficult to bridge the gap between early measurements (e.g., 1715) and modern helioseismic standards.
• Historical Solar Activity Reconstruction: The 1730s represent a "fault line" in solar cycle reconstructions due to the scarcity of sunspot data. Accurate documentation of solar phenomena (like prominences) from this era is essential to refine solar cycle timelines and understand the distribution of solar activity at high latitudes.

2. Methodology

The researchers employed a multi-disciplinary approach combining historical archival research with modern computational astrophysics:

• Archival Analysis: The team translated and analyzed Birger Wassenius’ unpublished manuscript (held in the Royal Academy Archives of Sweden), which contains detailed observations and sketches of the May 13, 1733 (Gregorian) total solar eclipse (TSE) that were not included in his previously published reports.
• Eclipse Reconstruction: Using the Sôma method, the authors reconstructed the local eclipse visibility at the observation site (Skansen Kronan, Göteborg). They utilized high-precision modern data, including NASA JPL DE 441 ephemeris and KAGUYA Mission lunar topography, to model the lunar-limb profile.
• Solar Radius Quantification: By matching Wassenius’ reported totality duration (128 seconds) with the calculated lunar-limb/solar-limb interaction, the authors constrained the solar radius.
• Heliographic Mapping: The authors calculated the apparent solar orientation (including $B_0$ and $P$ angles) at the time of maximum eclipse to map Wassenius’ qualitative descriptions of prominence locations onto a quantitative heliographic coordinate system.

3. Key Contributions

• Primary Source Recovery: The study provides the first scientific analysis of Wassenius’ unpublished sketches, which are among the earliest robust records of solar prominences.
• Chronological Data Filling: It provides a critical data point for the solar radius in the mid-18th century, helping to connect the 1715 observations to modern values.
• Solar Cycle Refinement: The analysis of high-latitude prominences offers a potential correction to the timing of Solar Cycle −1.

4. Results

• Solar Radius ($R_\odot$):
  • Absolute scale: $696,250 \pm 170$ km.
  • Apparent scale: $959.99 \pm 0.24''$.
  • This result is larger than the modern helioseismic standard ($959.34 \pm 0.22''$) but is consistent with the 1715 estimate, supporting the hypothesis of long-term centennial variability in the solar radius.
• Prominence Distribution:
  • Three prominences were identified at heliographic latitudes of approximately $+23.5 \pm 22.5^\circ$, $+66.5 \pm 22.5^\circ$, and $-68.5 \pm 22.5^\circ$.
  • The presence of prominences at such high latitudes ($>60^\circ$) is significant.
• Solar Cycle Context: The high-latitude observations contradict the sunspot butterfly diagrams of the 1725–1750 period, which suggest 1733 was a solar minimum.

5. Significance

The paper concludes that the 1733 observations are highly significant for two reasons:

1. The "Polar Rush" Hypothesis: If these high-latitude prominences are categorized as "polar rush" prominences (which typically appear during the rising phase of a solar cycle), the solar minimum must be re-dated to before May 1733. This would shift our understanding of the onset of Solar Cycle −1.
2. Magnetic Field Insights: Even if not part of a "polar rush," the existence of quiescent prominences at high latitudes suggests the presence of a polarity inversion line in the polar regions in early 1733, providing rare insight into the magnetic structure of the Sun during a period of low sunspot activity.