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

We realized that this [Kepler's] sunspot drawing should be able to tell us the location of the sunspot and indicate the solar cycle phase in 1607 as long as we managed to narrow down the observation point and time and reconstruct the tilt of the heliographic coordinates—meaning the positions of features on the Sun's surface—at that point in time.” //

German astronomer Gustav Spörer noted the steep decline in 1887 and 1889 papers, and his British colleagues, Edward and Annie Maunder, expanded on that work to study how the latitudes of sunspots changed over time. That period became known as the "Maunder Minimum." Spörer also came up with "Spörer's law," which holds that spots at the start of a cycle appear at higher latitudes in the Sun's northern hemisphere, moving to successively lower latitudes in the southern hemisphere as the cycle runs its course until a new cycle of sunspots begins in the higher latitudes.

But precisely how the solar cycle transitioned to the Maunder Minimum has been far from clear. //

"It is fascinating to see historical figures’ legacy records convey crucial scientific implications to modern scientists even centuries later," said co-author Sabrina Bechet of the Royal Observatory of Belgium. "I doubt if they could have imagined their records would benefit the scientific community much later, well after their deaths. 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."