The Sunspot Story

By Johna Till Johnson

Suspended lion face
Spilling at the centre
Of an unfurnished sky
How still you stand,
And how unaided
Single stalkless flower
You pour unrecompensed.

The eye sees you
Simplified by distance
Into an origin,
Your petalled head of flames
Continuously exploding. …

—Philip Larkin, Solar

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What’s the longest-running scientific experiment in history?

I bet you didn’t think of monitoring sunspots.

Sunspots (photo by NASA)

And I can’t definitively say it’s the longest-running experiment —there may be others I haven’t heard of yet. But the first recorded systematic observation of sunspots in the West was by astronomers Thomas Harriot, Johannes and David Fabricius  in 1610 (Chinese astronomers observed them as far back as 300 BC). Scientists have been monitoring sunspots ever since—which means that sunspots have been monitored for the past 405 years!

What are sunspots? From Wikipedia:

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere. …

Sunspots expand and contract as they move across the surface of the Sun and can be as small as 16 kilometers (10 mi) and as large as 160,000 kilometers (100,000 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope. They may also travel at relative speeds (“proper motions”) of a few hundred meters per second when they first emerge onto the solar photosphere.

Why do sunspots matter? It’s not entirely clear, though some theories are beginning to emerge. Because they wax and wane cyclically (the cycle is approximately 11 years), sunspots have  been historically associated with other cyclical, or pseudocyclical, phenomena on Earth: changes in women’s hemlines, the stock market, frequency of wars…

Those associations are probably a bit nutty, but a serious current hypothesis is that they affect the Earth’s climate. Specifically, there’s an inverse correlation between sunspots and the cosmic radiation that reaches the Earth.  That is,when there are more sunspots, there’s less cosmic radiation, and vice versa: fewer sunspots mean more cosmic radiation.

This matters because—as recent experiments at the European Organization for Nuclear Research (CERN) have shown—cosmic radiation enhances cloud formation, which cools things down. So when there are plenty of sunspots, there are fewer clouds, and Earth gets warmer; fewer sunspots mean more clouds and a cooler Earth.

As you can imagine, the degree of this effect is something of a hot topic (sorry!) among climate scientists, as sunspot activity hasn’t (yet) been factored into global warming models. Some good background articles are here and here.

But never mind why. What fascinates me is how it’s possible to conduct a multigenerational science experiment.

I learned most of the details from an entrancing paper written by (among others), one of the premier scientists researching sunspot activity, Leif Svalgaard, currently at Stanford University. The paper is here (PDF). I’ll warn you now that it’s a serious scientific paper, complete with formulas and dense mathematical analysis. The goal of the paper is as dry as it gets: to review the record of sunspots (both historic and modern) and arrive at a canonical data set that scientists can use to track long-term trends in sunspot activity.

However, Dr. Svalgaard and his co-authors are extremely good writers, and intermingled with the technical details is some of the personal history of the men who monitored sunspots for generations. There are some great photos, too, including one of notebooks from the 1660s.

As you’d imagine, there are several challenges with conducting observations across 400 years.  One is gaps in the record. Sunspot observers need to be extremely consistent and methodical: They need to examine the sun every day, preferably at the same time and from the same location, and record the results.

But Europe was a fairly busy place in the past four centuries, what with wars, famines, and dramatic regime changes—not to mention some pretty extreme weather. Unsurprisingly, there are multiple years for which there are no sunspot records (you can only imagine what the observers were doing at the time, say during the Napoleonic wars!).

There are bigger challenges, though. The fundamental problem,  it turns out, is maintaining consistency—both across generations, and across generations of technology. As telescopes improve, the number of visible sunspots naturally increases—so scientists need to find ways to compare measurements taken through a more powerful telescope with those taken through earlier, less powerful telescopes.

But even when technology remains consistent, there’s the problem of  comparing the observations of one scientist with those of another. In a nutshell, some scientists consistently record “low”, while others record “high”—due to differences in vision, or interpretation (is that one blob or two?).  Interestingly, it’s almost impossible to wring out the subjectivity from sunspot-counting—the scientist is inextricably linked with his research subject.

Most of the paper discusses mechanisms for doing this sort of normalization, so that today’s scientists can accurately rely on the blended data compiled by the dozens of individual observers over the centuries.

But what prompted me to write this post was an almost throwaway side note in the paper: Apparently in the late 20th and early 21st centuries, the measurements by one Italian scientist (who took readings over a whopping 55 years) had slowly declined, then began slowly increasing.

The explanation? As the paper notes drily,

A well-documented change in the observing routine took place progressively since 2005. As a careful preparation of the future replacement of S. Cortesi by Marco Cagnotti, the new Director of the Specola Observatory, the latter has been trained and is progressively taking over a larger fraction of the observations since 2005. This favors the following interpretation: over the last decades, the Wolf numbers [standard measure of sunspots] reported by S. Cortesi decreased slowly due to subtle aging effect and eyesight degradation. The Wolf numbers started to increase again with the progressive contribution by a younger observer, who now essentially records the same sunspot counts as S. Cortesi when he was younger, more than 30 years ago.

Talk about your research challenges! Even the same scientist practicing maximum consistency can’t be certain of getting consistent numbers.

I can’t figure out what possessed these men to methodically observe and record sunspots, day after day, year after year, generation after generation. But I’m really glad they did it!