Scientists Have Discovered a Detail in Kepler’s 400-Year-Old Sun Sketch That No One Noticed for Centuries

A 417-year-old drawing by Johannes Kepler has just helped solve a major solar puzzle. The 1607 sketch features sunspots that give scientists important insights into how the Sun behaved during the Maunder Minimum.

The Maunder Minimum, which lasted from 1645 to 1715, was a strange period when the Sun showed little to no sunspots. Sunspots are important because they indicate solar activity, like flares and magnetic activity. The mystery was that, despite the Sun’s regular cycles, this lull in activity lasted for decades.

Since telescopic observations were still new at the time, data about solar activity was scarce. Enter Kepler’s long-forgotten sketch, which is now giving scientists the information they’ve been missing.

Kepler’s Sunspot Sketch

While most people know Johannes Kepler for his laws of planetary motion, he also made significant observations of the Sun. In 1607, Kepler used a simple camera obscura to record sunspots.

At first, he thought the spots were caused by a transit of Mercury, but recent research, published in The Astrophysical Journal Letters, has shown they were actually sunspots, the first known instrumental observation of their kind. In 2024, a team from Nagoya University in Japan took a fresh look at the famous drawings.

“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,” explained Hisashi Hayakawa of Nagoya University, who led the study, in a statement.

The team found that Kepler’s observations likely took place at the end of Solar Cycle -13, just before the start of the Maunder Minimum. Prior to this, scientists had mostly relied on tree-ring data to track solar cycles, but this method had its flaws.

Kepler’s sunspot sketches give researchers a much clearer picture of the Sun’s activity around that time, allowing them to refine the timeline and understand the transition into the Maunder Minimum.

Solving the Puzzle of the Maunder Minimum

When he made his observations, telescopes were still new, and astronomers had only limited ways of studying the Sun. By analyzing his sunspot drawings, scientists now have a better understanding of solar activity during the early 1600s. According to Hayakawa, the lead researcher of the study:

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

While tree-ring data has been a valuable tool for studying solar activity, it can sometimes be unreliable. Tree rings record changes in cosmic rays, which vary with solar activity, but these readings can be inconsistent. Kepler’s observations give researchers a more accurate way to pinpoint when solar cycles were changing.

“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,” added Hayakawa.

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