This story is a collaboration with Biography.com.

Few things seem as immutable than the Sun—after all, it’s been fusing hydrogen for five billion years, and it’ll keep on ticking for about five billion more. But despite our star’s grand existence, the Sun’s magnetic field will experience several solar cycles in just one human lifetime.

These 11-year-long phases wax and wane between solar minimums and maximums, which are characterized by less or more solar activity (sunspots, flares, magnetic fields, etc.), respectively. Currently, we’re living through the 25th 11-year cycle since astronomers began closely tracking the Sun’s magnetic activity in 1755.

But there’s a mystery lingering at the heart of these seemingly tidy solar cycles.

From 1645 until 1715, the Sun experienced a sustained period of depressed solar activity known as the grand solar minimum, or the Maunder minimum; the phenomenon was named after the English astronomer who discovered it, Edward Walter Maunder. To fully understand this grand minimum, astronomers need data that predates the unexpected solar lull. But with the first telescope observations of the Sun only arriving a few decades before the Maunder minimum, data is hard to come by.

Good thing, then, that we have German astronomer Johannes Kepler.

Kepler is best known for his laws of planetary motion (as well as being the namesake of a very important NASA space telescope), but in 2024, scientists from Nagoya University in Japan—analyzing observations Kepler made of sunspots in 1607 using a camera obscura—announced that they believe this reinterpreted piece of data could help astronomers unravel the mysteries of the Maunder minimum.

And that reinterpretation is definitely needed, considering that Kepler originally thought he was witnessing a transit of Mercury. The results of the study were published in The Astrophysical Journal Letters.

“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,” Nagoya University’s Hisashi Hayakawa, the lead-author of the study, said in a press statement. “But this is the oldest sunspot sketch ever made with an instrumental observation and a projection.”

To correctly interpret Kepler’s original findings, Hayakawa and his team needed to narrow down when the observation was taken and reconstruct the positions of features on the Sun’s surface (what’s known as heliographic tilt). Previously, astronomers have relied on tree-ring observations—when the Sun is particularly active, solar winds and the solar magnetic field better protect Earth from galactic cosmic rays (which are engraved into tree rings as carbon-14). High solar activity means lower carbon-14 (and vice versa), and you can see those changing levels in the rings of trees.

But relying on only tree rings to understand solar cycles comes with some issues, as three separate observations place these cycles (in this case Solar Cycle -13 and -14) in the extremely short, normal, and even extremely long category. This is where Kepler’s observation comes in, and scientists discovered that this 417-year-old observation likely occurred at the tail-end Solar Cycle -13, rather than beginning of -14.

Later telescopic observations also detail how Kepler’s drawing likely points to a typical transition from the previous solar cycle, and the team could now significantly narrow down when that transition took place—between 1607 and 1610. The end result? During this time, the Sun exhibited a typical solar cycle.

“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,” Hayakawa said in a press statement. “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.”