Kepler's overlooked Sun studies from 17th century solve modern solar mystery (2024)

NAGOYA, Japan — The Sun has long puzzled scientists with its cyclical behavior. Now, a team of international researchers has found an unexpected clue to its past in the writings of a 17th-century astronomer. Their study, published in The Astrophysical Journal Letters, shows how Johannes Kepler’s observations are rewriting our understanding of solar cycles.

Most people know Kepler for his laws of planetary motion, but few realize he was also an early observer of sunspots. Sunspots are dark areas on the Sun’s surface caused by intense magnetic activity. They appear and disappear in cycles, typically lasting about 11 years. These cycles are important because they affect space weather, which can impact satellites, power grids, and even climate patterns on Earth.

Until now, scientists believed that reliable sunspot observations began in 1610 with the invention of the telescope. However, this new study reveals that Kepler made detailed sunspot observations three years earlier, in 1607, using a method called camera obscura. This technique projects an image of the Sun through a small hole onto a flat surface, allowing safe observation of the solar disk.

Kepler’s observations are significant because they help pinpoint the beginning of a solar cycle that occurred just before the start of the well-documented telescopic era. This particular cycle is crucial for understanding the transition between regular solar activity and an unusual period known as the Maunder Minimum, which lasted from about 1645 to 1715. During this time, sunspots were extremely rare, and some scientists believe it coincided with a period of cooler global temperatures called the Little Ice Age.

The research team, led by Hisashi Hayakawa from Nagoya University in Japan, carefully analyzed Kepler’s original Latin texts and drawings. They discovered that Kepler had observed a large sunspot group on May 28, 1607. By calculating the position of this sunspot on the Sun’s surface, the researchers determined that it was likely part of the tail end of a solar cycle, rather than the beginning of a new one.

“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,” says Hayakawa. “But this is the oldest sunspot sketch ever made with an instrumental observation and a projection.”

The study also sheds light on the debate surrounding solar cycle durations during this period. Some reconstructions based on tree-ring data had suggested extremely short or long solar cycles around this time. However, Kepler’s observation supports the idea that solar cycles were operating normally in the early 1600s, with the transition to the Maunder Minimum occurring more gradually than previously thought.

Thomas Teague, an observer for the WDC SILSO and a member of the research team, explains verbatim: “This shows a typical transition from the preceding solar cycle to the following cycle, in accordance with Spörer’s law,” referring to the German astronomer Gustav Spörer who described a migration of sunspots from higher to lower latitudes during a solar cycle.

The study highlights the value of historical astronomical observations. Even though Kepler didn’t have a telescope, his careful records provide valuable data points that help fill gaps in our understanding of solar activity. This underscores the importance of preserving and studying historical scientific documents, as they may contain overlooked information that can inform current research.

“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. I doubt if they could have imagined their records would benefit the scientific community much later, well after their deaths,” says Sabrina Bechet, a researcher at the Royal Observatory of Belgium. “We still have a lot to learn from these historical figures, apart from the history of science itself. In the case of Kepler, we are standing on the shoulders of a scientific giant.”

As we face the challenges of climate change today, understanding past solar activity and its potential effects on Earth’s climate becomes increasingly important. While the connection between solar cycles and climate is complex and still debated, studies like this one provide crucial pieces of the puzzle. By better understanding solar behavior in the past, scientists can improve their models of solar activity and its potential impacts on our planet in the future.