How Kepler's 400-Year-Old Sunspot Sketches Solved a Modern Mystery

The sharp decline in sunspot activity in the 17th century has long puzzled astronomers.

A team of Japanese and Belgian astronomers have re-examined sunspot drawings made by 17th-century astronomer Johannes Kepler using modern analytical techniques, solving a long-standing mystery about the solar cycles of that period, according to a recent paper articlepublished in The Astrophysical Journal Letters.

The exact identity of who first observed sunspots was hotly debated in the early 17th century. We now know that ancient Chinese astronomers between 364 and 28 BCE observed them and included them in their official records. A Benedictine monk in 807 thought he had observed Mercury passing in front of the Sun when he had actually observed a sunspot; similar misinterpretations were common in the 12th century. (An English monk made the first known drawings of sunspots in December 1128.)

The English astronomer Thomas Harriot made the first telescopic observations of sunspots in late 1610 and recorded them in his notebooks, as did Galileo around the same time, although the latter did not publish a scientific paper on sunspots (accompanied by drawings) until 1613. Galileo also argued that sunspots were not satellites of the Sun, as some believed, but rather clouds in the atmosphere or on the surface of the Sun. But he was not the first to suggest this; that credit goes to the Dutch astronomer Johannes Fabricus, who published a scientific treatise on sunspots in 1611.

Kepler read and admired this treatise because in 1607 he had made his own camera obscura observations of sunspots (published in a 1609 treatise), which he initially mistook for a transit of Mercury. In 1618 he refuted this report, concluding that he had in fact seen a group of sunspots. Kepler made his solar drawings based on observations made both in his own home and in the workshop of the court mechanic Justus Burga in Prague. In the former case he reported “a small spot about the size of a small fly,” in the latter “a small spot of deep darkness toward the center… resembling in size and appearance a thin flea.”

The long-standing debate that is the subject of this latest paper concerns the period from 1645 to 1715, during which, despite the best efforts of astronomers, very few sunspot observations were recorded. This was a unique event in the history of astronomy. Although only 59 sunspots were observed during this time—compared to 40,000 to 50,000 spots in the same period today—astronomers were still able to determine that sunspots appeared to occur in an 11-year cycle.

German astronomer Gustav Spörer noted the sharp decline in papers in 1887 and 1889, and his British colleagues Edward and Annie Maunder expanded on this work by studying how sunspot latitudes changed over time. This period became known as the “Maunder Minimum.” Spörer also came up with “Spoerer's Law,” which states that sunspots begin at higher latitudes in the Sun's northern hemisphere and move to lower and lower latitudes in the southern hemisphere as the cycle progresses, until a new sunspot cycle begins at higher latitudes.

But exactly how the solar cycle transitioned to the Maunder Minimum is still unclear. Tree-ring reconstructions provide conflicting data. For example, one such reconstruction suggests that the gradual transition was preceded by either an extremely short solar cycle of about five years or an extremely long solar cycle of about 16 years. Another tree-ring reconstruction suggests that the solar cycle was of normal length, 11 years.

Independent observational data may help resolve this discrepancy. So Hisashi Hayakawa of Nagoya University in Japan and co-authors turned to Kepler's sunspot images, which predate existing telescopic observations by several years, for additional clues.

Kepler's Quantitative Assessment

“Because this record was not a telescopic observation, it was discussed only in the context of the history of science and was not used to quantitatively analyze solar cycles in the 17th century,” Hayakawa says. “But it is the oldest sunspot drawing ever made using instrumental observation and projection. We realized that this sunspot drawing could pinpoint its location and the phase of the solar cycle in 1607 if only we could narrow down the point and time of observation and reconstruct the tilt of the heliographic coordinates — that is, the positions of objects on the surface of the Sun — at that point in time.”

Kepler placed both sets of sunspots in the lower left quadrant of his drawings. Hayakawa et al. “projected” the drawings (which were upside down, given his camera obscura observations) and compensated for the tilt by comparing Kepler's records with modern data for clarity. They found that the sunspots were located at a lower latitude. This means that Kepler's famous schematic drawing, presented in his book, contradicts both the camera obscura images and the astronomer's original text. The authors suggest that the draftsman who created the illustration may have made a mistake.

Using Spörer’s law, the team determined that the sunspot group originated at the end of the previous solar cycle (Solar Cycle 13), rather than the beginning of the new one (Solar Cycle 14). Later telescope observations during the new cycle revealed that the sunspots were at higher latitudes, consistent with Spörer’s law. They were also able to narrow the timing of the transition between the two cycles to between 1607 and 1610, indicating that the previous solar cycle was regular in length, rather than being extremely short or extremely long.

“It’s fascinating to see how the legacy of historical figures’ writings continues to inform modern scientists centuries later,” says study co-author Sabrina Bechet of the Royal Observatory of Belgium. “Little did they know that their writings would be useful to the scientific community much later, after their death. We have much to learn from these historical figures beyond the history of science itself. In the case of Kepler, we are standing on the shoulders of a scientific giant.”