Their findings, published in Astrophysical Journal Letters, provide crucial information on the length of solar cycles at the dawn of the 17th century. This period marks the transition from regular solar cycles to the grand solar minimum, known as the Maunder Minimum (1645-1715), a prolonged phase of low sunspot activity significant for understanding solar behavior and its terrestrial impacts.

Kepler, celebrated for his contributions to astronomy and mathematics, made some of the earliest instrumentally recorded observations of solar activity using a camera obscura. In May 1607, he documented what he initially thought was a transit of Mercury but was later identified as a sunspot group. These dark areas on the Sun, caused by intense magnetic activity, follow cycles that influence solar radiation and space weather.

Hisashi Hayakawa, the study's lead author, remarked, "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. But this is the oldest sunspot sketch ever made with an instrumental observation and a projection."

Hayakawa explained, "We realized that this 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."

The early 17th century was a crucial era for solar observations, marking the beginning of recorded sunspot observations and the shift to the Maunder Minimum. The gradual transition from regular solar cycles to this grand minimum is not fully understood. Some reconstructions based on tree-ring data suggest a mix of unusually short and long solar cycles preceding the minimum, while others indicate normal cycle durations.

Hayakawa noted, "If true, this would indeed be interesting. However, another tree-ring-based reconstruction indicated a sequence of solar cycles with normal durations. Then, which reconstruction should we trust? It is extremely important to check these reconstructions with independent - preferably observational - records."

Kepler's sunspot record provides a vital observational reference. The team discovered four key points from their analysis: First, they corrected the positioning of Kepler's sunspot drawings, finding them at low heliographic latitudes. This contradicts Kepler's schematic drawing, which inaccurately placed the sunspot. Second, using Sporer's law and modern statistics, they identified the sunspot group at the end of solar cycle -13, not the start of cycle -14. Third, their findings differ from later telescopic observations showing sunspots at higher latitudes, consistent with the transition between solar cycles as described by Sporer's law. Lastly, they determined the transition between cycles -14 and -13 occurred between 1607 and 1610, supporting a regular duration for cycle -13.

Hayakawa emphasized Kepler's lasting impact, "Kepler's legacy extends beyond his observational prowess; it informs ongoing debates about the transition from regular solar cycles to the Maunder Minimum, a period of extremely reduced solar activity and anomalous hemispheric asymmetry between 1645 and 1715. 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 added, "Kepler contributed many historical benchmarks in astronomy and physics in the 17th century, leaving his legacy even in the space age. Here, we add to that by showing that 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."

Sabrina Bechet from the Royal Observatory of Belgium stated, "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. 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."

Research Report:Analyses of Johannes Kepler's Sunspot Drawings in 1607: A Revised Scenario for the Solar Cycles in the Early 17th Century

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