Johannes Kepler thought he had drawn Mercury orbiting the Sun. What he actually captured is the solution to a solar mystery

(CNN) – 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.

Although everything in the solar system revolves around the Sun, scientists have yet to unravel many of its secrets.

However, studying the Sun's variability over time, including the solar cycle, could answer some of the oldest questions about the fiery orb and how it changes.

Some of these questions revolve around solar activity in the 17th century, a crucial period for the study of the Sun.

In 1610, astronomers first observed sunspots with telescopes. At the time, the Sun was undergoing an unusual transition to a long period of weakened activity. And Kepler's sketches, long ignored because they were drawings rather than telescopic observations, could provide crucial historical data.

A new one study which recreates the circumstances in which Kepler made his drawings was published on July 25 in the academic journal The letters of the astrophysical journal.

“Kepler provided many landmarks in astronomy and physics in the 17th century, leaving its legacy even in the space age,” Hisashi Hayakawa, lead author of the study and an adjunct professor at Nagoya University’s Space-Earth Environment Research Institute, said in a statement.

“In this study, we have shown that Kepler’s sunspot records predate the telescopic sunspot records of 1610 by several years. His sunspot sketches serve as a testament to his scientific acumen and perseverance in the face of technological limitations.”

The sun undergoes an 11-year cycle of waxing and waning activity, known as the solar cycle. Scientists currently believe the sun is reaching or approaching 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 be as large as Earth or larger, are driven by the Sun's strong, ever-changing magnetic fields.

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

But simply trying to observe the Sun was a difficult feat centuries ago.

The sunspots were visible to the naked eye through fog, haze, wildfire smoke or near sunrise or sunset, when the atmosphere helped dim the sun's glare, said Mark Miesch, a 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 a device called a camera obscura, which uses a small hole in the wall of the instrument to project an image of the Sun onto a sheet of paper and draw the features he observed. Kepler mistakenly believed he had captured Mercury orbiting in front of the Sun in May 1607, but 11 years later he recanted and determined that he had observed a group of sunspots.

“As 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 analysis of solar cycles in the 17th century,” Hayakawa said.

“But this is the oldest drawing of a sunspot made using instrumental observation and projection. We realized that this sunspot drawing should be able to tell us the location of the sunspot and indicate the phase of the solar cycle in 1607 as long as we could constrain the point and time of observation and reconstruct the inclination of the heliographic coordinates – that is, the positions of features on the Sun’s surface – at that time.”

Sunspots aren't the only way scientists understand changes on the Sun. Variations in the Sun's magnetic field regulate the movement of high-energy particles, called cosmic rays, through space, Miesch explains.

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

“Over time, this carbon is incorporated into plants and animals – even ourselves,” Miesch explains. “Tree rings offer a unique opportunity to follow the evolution of carbon from year to year. Some ancient tree rings can date back thousands of years. Carbon isotopes and other elements can similarly be traced through air bubbles trapped in glacial ice cores.”

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

These data have 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, with the few that were observed appearing only in the solar southern hemisphere. The underlying mechanism of the Grand Solar Minimum remains a matter of debate among astronomers today, particularly as they try to figure out when and how it might occur in centuries to come.

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

AND previous analysis from tree rings suggested that a short solar cycle, Solar Cycle minus 14, only lasted about five years and led to an extremely long 16-year solar cycle, known as Solar Cycle minus 13.

“If true, this would be really interesting,” Hayakawa said. “However, another tree-ring reconstruction indicated a sequence of solar cycles with normal lengths (11 years). So 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 to narrow down the time range and locations during which Kepler made his observations.

Hayakawa then visited sites in Prague, such as Kepler's residence at the French Crown and the workshop of the court mechanic Justus Burgi, to better understand the topography from which Kepler observed sunspots.

Modern data tools allowed Hayakawa and his colleagues to calculate the sunspot's inclination and determine its location on the Sun. They also applied the Spörer's lawfirst observed by the English amateur astronomer Richard Christopher Carrington and later developed by the German astronomer Gustav Spörer, who described a migration of sunspots from higher to lower latitudes during a solar cycle.

The team of researchers determined that the sunspot group observed by Kepler belonged to the tail of Solar Cycle minus 14 and not the beginning of Solar Cycle minus 13.

The results 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 probably began between 1607 and 1610.

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

Since the longest solar cycle ever recorded in the past three centuries lasted 14 years, it is time to find another scientific precursor to the Maunder Minimum, Hayakawa said.

There is still much to learn from historical figures like Kepler, says Sabrina Bechet, co-author of the study and a researcher at the Royal Observatory of Belgium.

“As one of my colleagues told me, it is fascinating to see how records of the legacy of historical figures 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.”

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

“By placing Kepler’s findings within broader reconstructions of solar activity, scientists gain crucial context for interpreting changes in solar behavior during this crucial period that marked the transition from regular solar cycles to the Grand Solar Minimum,” he said.

Miesch called the new study “impressive work” and an example of detective work that draws new insights from the historical record.

“The long history of sunspot observations provides a link across the centuries to generations of astronomers who have looked upon the Sun with a reverence and a curiosity that has grown 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 now more important than they could ever have imagined, as our technological society becomes increasingly vulnerable to the eternal ups and downs of solar activity.”