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The 315-Year-Old Science Experiment

How counting sunspots unites the past and future of science.

The most arrogant astronomer in Switzerland in the mid-20th century was a solar physicist named Max Waldmeier. Colleagues were so…By Jonathon Keats

The most arrogant astronomer in Switzerland in the mid-20th century was a solar physicist named Max Waldmeier. Colleagues were so relieved when he retired in 1980 that they nearly retired the initiative he led as director of the Zurich Observatory. Waldmeier was in charge of a practice that dated back to Galileo and remains one of the longest continuous scientific practice in history: counting sunspots.

The Zurich Observatory was the world capital for tallying sunspots: cool dark areas on the sun’s surface where the circulation of internal heat is dampened by magnetic fields. Since the 19th century, astronomers had correlated sunspots with solar outbursts that could disrupt life on Earth. Today scientists know the spots mark areas in the sun that generate colossal electromagnetic fields that can interfere with everything from the Global Positioning System to electricity grids to the chemical makeup of our atmosphere.

What alienated Waldmeier’s potential Swiss successors was his hostility toward methods other than his own. In the space age, he insisted on counting sunspots by eye, using a Fraunhofer refracting telescope, named after its 18th-century inventor, installed by the first Zurich Observatory director, Rudolf Wolf, in 1849. (With Waldmeier’s legacy uncertain, his assistant walked off with the Fraunhofer telescope and installed it in his garden.) Automated observation—and solar monitoring by satellite—seemed like obvious improvements, far less subjective than squinting.

Yet for all the animosity toward Waldmeier, his method persisted. Sunspots appear in cycles. Their number steadily increases over a period of approximately 11 years, followed by about 11 years of decrease. Waldmeier understood the interpretation cannot be hurried because of the inherent slowness of the cycle itself. “You cannot speed up the process,” says astronomer Frederic Clette, director of the Solar Influences Data Analysis Center, based at the Royal Observatory of Belgium. “If you want to understand the sun, you must keep a record of the cycle continuously over long durations.”

Astronomer William Herschel believed sunspots were portholes into a dark subsolar world where people lived beneath the sun’s radiant sheath.

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The best way to ensure data remains consistent, explains Clette, is to employ a method of observation that links the past and present. In contrast to most new science, which progresses in tandem with technological developments, the most stable apparatus for detecting change in the star that gives us life is the human brain and eye.

“Modern techniques and equipment are powerful, but the technologies span over only a few solar cycles, so they don’t show how cycles differ over centuries,” says Clette, who is the custodian of the worldwide sunspot count, begun by Wolf in Zurich, and now known as the International Sunspot number. Under Clette’s watch, blemishes are still counted by eye. “When we count by eye, what we observe now can be connected to what was observed in the distant past.”

It’s a remarkable story, says Clette. One of the most enduring scientific methods is simply observing. “It’s a long and systematic evolution of accumulating information that has led to an understanding of the sunspot phenomenon, and the jewel on the crown, the ability to predict the future.”

DON’T FIX IT IF IT AIN’T BROKE: Seen here, the Fraunhofer refracting telescope, named after its 18th-century inventor, was employed by solar physicists to count sunspots well into the 20th century.University Observatory Munich


The observation of sunspots predates modern astronomy by at least three millennia. Since the sun was central to several ancient religions, any blemish was sure to be seen as significant. For ancient Africans living on the Zambezi River, sunspots were mud spattered in the face of the sun by a jealous moon. Ancient Chinese saw sunspots as the building blocks of a floating palace or even brushstrokes signifying the character for king. Virgil was more practical. “When [the sun] has checkered with spots his early dawn ... beware of showers,” he warned in his Georgics.

Galileo studied sunspots more scientifically, seeing them as useful markings to calibrate his study of the solar disc. From careful telescopic observation of their daily changes in appearance, he correctly deduced that the sun was spherical and rotated on its own axis, carrying the mutable blemishes with it. But to his eyes, and those of other early astronomers, the meandering of sunspots seemed random. That left plenty of opportunity for speculation: Philosopher Rene Descartes thought the spots were oceans of primordial scum. Astronomer William Herschel believed they were portholes into a dark subsolar world where people lived beneath the sun’s radiant sheath.

Yet there was one amateur astronomer who was simply content to watch the sky and document what he saw. An apothecary by trade, Heinrich Schwabe started observing the sun in 1826, and did so continuously, more than 300 days a year, for four decades. Initially he was searching for undiscovered planets inside Mercury’s orbit. Finding nothing solid, his focus gradually shifted to the speckled solar surface.

By 1844, having counted tens of thousands of spots, Schwabe grew convinced that there was a cycle to the blotchiness: The number of sunspots seemed to wax and wane every 10 years. He had no explanation, but reckoned that others might learn from his observation, so he published a page-long note in Astronomische Nachrichten. His paper was read by Rudolf Wolf, the 30-year-old director of the Bern Observatory. When Wolf took over as director of the Zurich Observatory in 1864, he decided to make the sunspot cycle a focus of study.

Wolf was not content to count only forward in time. To determine whether there truly was a cycle, and to get its true measure, he shrewdly sought to collect past data—starting with Schwabe’s—and to integrate it with his own daily observations.

The trouble was the figures didn’t sync. Their numbers didn’t match even when they counted on the same day, as they did thousands of times between 1849 and Schwabe’s final count in 1868. Wolf’s Fraunhofer telescope was considerably more powerful than Schwabe’s old instrument, revealing that many of Schwabe’s spots were actually clusters. To compensate, Wolf made two crucial decisions. The first was to censor his count—tallying clusters instead of individual spots—reasoning that the relative amount of sunspot activity was what really mattered. Wolf’s second important decision was to establish a ratio between himself and Schwabe by comparing their counts on days that both men observed the sun. That gave him a coefficient he dubbed k, a multiple that could be applied to all of Schwabe’s pre-1849 observations, statistically aligning them with Wolf’s newer data.

Climatologists want to know whether little ice ages are caused by periods when the sun is spotless, as was the case in the 18th century.

The coefficient permitted something even more remarkable. By a series of coincidental overlaps in observation, Wolf could work his way back from Schwabe to establish k coefficients for other scientists, and reliably extend his sunspot data all the way to 1700. Wolf then created a continental network of sunspot counters, and their daily tallies, ranging from zero to a couple hundred, became one of the most reliable datasets in astronomy.

The data showed that Schwabe was right about the sunspot cycle, but not its duration. At first Wolf recalculated the period to 11 years, which led him to believe he’d discovered the cause: Eleven years is the time it takes Jupiter to orbit the sun. Yet the more sunspot cycles he collected, the less plausible his correlation seemed. Some cycles were as long as 14 years. Others were as short as nine. Since Jupiter’s orbital period was invariant, he had to concede defeat.

He kept counting, confident that someone would figure out the sunspot mechanism given enough data. He counted all the way up to his death in 1893. By then his assistant, Alfred Wolfer, had been counting alongside him for 17 years. Their k coefficient made the observational transition seamless to subsequent directors at the Zurich Observatory, including the haughty Waldmeier, who developed an evolutionary classification of sunspots, and method of forecasting geomagnetic storms, that greatly advanced solar science.

see spot sun: This stunning image of a sunspot signals where magnetism has suppressed the movement of heat through the sun, a process known as solar convection. Sunspots mark areas that spark colossal flares that affect GPS and electricity grids on Earth.SST, Royal Swedish Academy of Sciences


So why are there periods of dark spottiness followed by periods when the sun is clear? “The truth is that we still don’t know for sure what causes the periodicity,” admits Clette. Even with 315 years of sunspot data, the inner workings of the sunspot cycle have yet to be illuminated in full.

Still, a lot of progress has been made since Schwabe’s era, notably on the impact of the solar outbursts. In 1859, two amateur astronomers in Wolf’s observational network noticed two bright flares inside a cluster of sunspots. Over the following days, telegraph service was disrupted and auroras could be seen across Europe. Several episodes convinced scientists that there was a connection, the explanation for which came in 1908 when astronomer George Ellery Hale used a spectroscope to determine that sunspots are magnetic. (Magnetism subtly interferes with the color spectrum.) The sun’s dark blemishes could finally be understood. They weren’t primordial scum or signs of solar habitation, but areas where magnetism suppressed the movement of heat through the sun, a process known as solar convection.

Today, thanks to solar physics, we know the sunspot cycle is driven by the rotational motion of plasma within the spinning sun. Because the plasma is electrically charged, and layers of plasma rotate at different speeds, the solar sphere behaves like a dynamo, producing electromagnetic fields that are thousands of times stronger than Earth’s polar magnetism. The circulation of plasma that creates a solar dynamo is now being modeled on supercomputers. Centuries of sunspot data help scientists to refine and validate those models by running simulations, seeing which models most closely match the varying periodicity of successive cycles. And the more perfect models become, the better the sunspot cycle itself will be understood.

The urgency for counting sunspots, explains Clette, has only increased as we’ve moved from an era of telegraphs to satellites. “The sunspot number helps establish the trend over the coming months and years for predicting the frequency and magnitude of disturbances,” he says. The Royal Observatory of Belgium receives regular requests for data from telecommunications and power companies. Commercial airlines also depend on sunspot trends because solar magnetism affects the rate at which radio waves pass through the ionosphere, warping GPS coordinates. If solar weather is trending toward storminess, pilots will shift their attention to alternative navigational instruments.

There also are more speculative correlations between sunspots and life on Earth. Medical researchers are keen to find connections between solar magnetism and cancer. Economists look for relationships between sunspot cycles and agriculture. And climatologists want to know whether little ice ages are caused by periods of “grand minimum”—when the sun is almost spotless—as was the case in the early 18th century. (Period paintings show people ice skating on the Thames and Venice’s lagoons.)

Progress in climatology is especially compelling. Solar radiation is known to change the chemistry of the upper atmosphere, and sunspots are known to modulate the intensity of different wavelengths—from infrared to X-rays—bombarding our planet. By linking the sunspot number to variations in the solar spectrum, climatologists will soon be able to deduce the spectral signature of the sun during the 18th-century grand minimum.

It’s an application that Wolf could never have anticipated, and a lesson to would-be Wolfs present and future: Solving one of the most pressing problems in contemporary science—how the global climate changes—will depend on data collected long before the problem was known. “I think it’s the essence of scientific research when you observe a new phenomenon that you cannot understand,” says Clette. “It’s like discovering a new territory. You know that new knowledge will be gained, even if it comes from different directions than you expect.”

Explaining the sunspot cycle would be the ultimate vindication of Wolf’s multi-century gambit. Yet in his role as custodian of sunspots, Clette is as jubilant about another breakthrough: He has recently established contact with the man who inherited Wolf’s instruments from Waldmeier’s renegade assistant. Observations from the old Fraunhofer telescope are once again contributing to the international sunspot count.

Clette’s elation is not at all sentimental, but celebrates Wolf’s central role in making sunspot counting consistent. “I’ve been able to establish the k coefficient on the telescope,” he says. “It matches perfectly what Wolf established in the 19th century—and keep in mind that the present observer is not Wolf. The matching k coefficient is an indication that the eye-brain system hasn’t evolved in the past couple centuries.”

And if the past couple centuries are a good measure, then simple observation will be viable far into the future. The sunspot count can be a model for any study that requires ultra-long-term data collection, such as the subtle changes in an ancient star’s behavior in the thousands of years before a supernova. Spanning tens or hundreds of generations, a supernova study would make sunspot counting seem as quick as scoring a baseball game.

This experiment in deep time will be an epic challenge. It will depend on statistical cleverness worthy of Wolf and stubborn traditionalism worthy of Waldmeier. Yet to reach its fullest potential, it will take the placid mindset of Schwabe, who didn’t need to know what would eventually be found in his data, only that there was merit in observing.


Jonathon Keats is a writer, artist and experimental philosopher based in San Francisco and Northern Italy. His new book, on the legacy of Buckminster Fuller, will be published by Oxford University Press next year.

The lead photocollage was created from an image from NASA.

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