German astronomer Johannes Kepler made sketches of sunspots in 1607 from his observations of the sun’s surface — and centuries later, the pioneering drawings are helping scientists solve a solar mystery.

Even though everything in the solar system revolves around the sun, scientists have yet to unlock many of the star’s secrets.

However, studying the variability of the sun over time, including the solar cycle, could answer some of the most longstanding questions about the fiery orb and how it changes.

Some of those questions revolve around solar activity in the 17th century, which was a pivotal time for studying the sun.

Astronomers observed sunspots with telescopes for the first time in 1610. At the same time, the sun was making an unusual transition into an extended period of weakened activity. And Kepler’s long disregarded sketches, overlooked because they were drawings rather than telescopic observations, could provide crucial historical insights.

A new study that recreates the circumstances during which Kepler made his drawings appeared on July 25 in The Astrophysical Journal Letters.

“Kepler contributed many historical benchmarks in astronomy and physics in the 17th century, leaving his legacy even in the space age,” said lead study author Hisashi Hayakawa, assistant professor at Nagoya University’s Institute for Space-Earth Environmental Research, in a statement.

“Here, we add to that by showing that Kepler’s sunspot records predate the existing telescopic sunspot records from 1610 by several years. His sunspot sketches serve as a testament to his scientific acumen and perseverance in the face of technological constraints.”

The sun’s tumultuous activity

The sun experiences an 11-year cycle of waxing and waning activity, known as the solar cycle. Currently, scientists believe that the sun is reaching or nearing solar maximum, the annual peak of its activity for the current solar cycle, called Solar Cycle 25.

Solar maximum is typically associated with an increase in the number of sunspots visible on the sun’s surface. These dark regions, some of which can reach the size of Earth or larger, are driven by the sun’s strong and constantly shifting magnetic fields.

Today, scientists track solar activity using data from ground and space-based observatories, magnetic maps of the solar surface, and ultraviolet observations of the sun’s outer atmosphere.

But just trying to observe the sun was a difficult feat centuries ago.

Sunspots were observed with the naked eye through fog, haze, wildfire smoke, or near sunrise or sundown when the atmosphere helped to dim the sun’s brightness, said Mark Miesch, research scientist at the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center in Boulder, Colorado. Miesch was not involved in the new research.

Kepler used an apparatus called a camera obscura, which utilized a small hole in the wall of the instrument to project the sun’s image on a sheet of paper and sketched the features he observed. Kepler mistakenly believed he had captured Mercury moving in orbit across the sun in May 1607, but he later retracted his report 11 years later and determined he had observed a sunspot group.

“Since this record was not a telescopic observation, it has only been discussed in the context of the history of science and had not been used for quantitative analyses for the solar cycles in the 17th century,” Hayakawa said.

“But this is the oldest sunspot sketch ever made with an instrumental observation and a projection. We realized that this sunspot drawing should be able to tell us the location of the sunspot and indicate the solar cycle phase in 1607 as long as we managed to narrow down the observation point and time and reconstruct the tilt of the heliographic coordinates — meaning the positions of features on the Sun’s surface — at that point in time.”

A grand solar minimum

Sunspots aren’t the only way scientists can understand changes in the sun. Variations within the sun’s magnetic field regulate the movement of high-energy particles, called cosmic rays, through space, Miesch said.

When cosmic rays hit Earth’s atmosphere, they can change its chemistry, including the balance of carbon.

“In time, this carbon is incorporated into plants and animals, even ourselves,” Miesch said. “Tree rings provide a unique opportunity to trace the change in carbon from one year to the next. Some rings in ancient trees can be traced back for thousands of years. Isotopes of carbon and other elements can similarly be traced through air bubbles trapped in glacial ice cores.”

The carbon isotopes trapped in tree rings and ice cores have been used to contextualize ancient sunspot observations and extend our understanding of solar activity before sunspot observations occurred, Miesch said.

Such data has been used to help astronomers understand the Maunder Minimum, a period of extremely weak and abnormal solar cycles between 1645 and 1715. During this so-called grand solar minimum, sunspots virtually disappeared, and the few that were observed appeared only in the southern solar hemisphere. The background mechanism of the grand solar minimum is still debated by astronomers today, especially as they try to work out when and if it could occur in future centuries.

But astronomers agree that the pattern of solar activity shifted from regular cycles to the grand minimum gradually.

A previous tree-ring analysis suggested a brief solar cycle, Solar Cycle minus 14, was only about five years long and led to an extremely long solar cycle of 16 years, known as Solar Cycle minus 13.

“If true, this would indeed be interesting,” Hayakawa said. “However, another tree-ring-based reconstruction indicated a sequence of solar cycles with normal durations (11 years). Then, which reconstruction should we trust? It is extremely important to check these reconstructions with independent — preferably observational — records.”

So, he turned to Kepler’s sketches.

Hayakawa and his colleagues translated Kepler’s original report, written in Latin, to understand the exact orientation of his sunspot sketches, as well as narrow down the time range and locations during which Kepler made the observations.

Hayakawa then visited sites in Prague, including Kepler’s residence at the French Crown and the workshop of court mechanic Justus Burgi, to better understand the topography from where Kepler saw the sunspots.

Modern data tools enabled Hayakawa and his colleagues to calculate the inclination of the sunspot and determine its location on the sun. They also applied Spörer’s law, first observed by English amateur astronomer Richard Christopher Carrington but further developed by German astronomer Gustav Spörer, who described a migration of sunspots from higher to lower latitudes during a solar cycle.

The research team determined that the sunspot group observed by Kepler belonged to the tail-end of Solar Cycle minus 14 rather than the beginning of Solar Cycle minus 13.

The findings support the idea that Solar Cycle minus 13 had a regular duration of 11 years rather than 16 years. The researchers were also able to estimate that Solar Cycle minus 13 likely began between 1607 and 1610.

“This shows a typical transition from the preceding solar cycle to the following cycle, in accordance with Spörer’s law,” said study coauthor Thomas Teague, an observer at the Solar Influences Data Analysis Center at the Royal Observatory of Belgium, in a statement.

Given that the longest solar cycle ever recorded within the past three centuries lasted for 14 years, it’s tim to find another scientific precursor for the Maunder Minimum, Hayakawa said.

Kepler’s enduring legacy

There is still much to learn from historical figures like Kepler, said study coauthor Sabrina Bechet, a researcher at the Royal Observatory of Belgium.

“As one of my colleagues told me, it is fascinating to see historical figures’ legacy records convey crucial scientific implications to modern scientists even centuries later,” Bechet said. “In the case of Kepler, we are standing on the shoulders of a scientific giant.”

Kepler’s sketches are helping to inform ongoing debates about the solar cycles that led up to the Maunder Minimum, which could also help astronomers model the conditions before the event, Hayakawa said.

“By situating Kepler’s findings within broader solar activity reconstructions, scientists gain crucial context for interpreting changes in solar behaviour in this pivotal period marking a transition from regular solar cycles to the grand solar minimum,” he said.

Miesch called the new study an “impressive piece of work” and an example of detective work that teases new insights from historical records.

“The long history of sunspot observations provides a link through the ages to generations of astronomers who have regarded the sun with reverence and curiosity that has progressed from superstition to scientific scrutiny to understanding. It is inspiring to see that astronomers of the past continue to contribute to scientific discovery. And their efforts are more important now than they could have ever imagined, as our technological society becomes increasingly vulnerable to the timeless waxing and waning of solar activity.”