Kepler’s 1607 Sunspot Sketch Clarifies Maunder Minimum Onset

The Maunder Minimum lasted from 1645 to 1715, a span of about 70 years when sunspots vanished and solar activity dropped sharply. Sunspots are dark patches that signal strong magnetic fields and can precede flares. Before telescopes became common, astronomers had few ways to spot them, leaving a gap in the solar record.

Johannes Kepler used a camera obscura in 1607 to draw sunspots, thinking at first they were a transit of Mercury. A team from Nagoya University re‑examined the sketch and, in *The Astrophysical Journal Letters*, confirmed the marks were genuine sunspots. Hisashi Hayakawa, the lead researcher, noted that Kepler’s non‑telescopic record had previously been treated only as a historical curiosity and never used for quantitative solar‑cycle work.

By anchoring a concrete sunspot count to 1607, scientists can tighten reconstructions of solar cycles leading into the Maunder Minimum. This reduces reliance on tree‑ring data, which records cosmic‑ray fluctuations but can be noisy and uncertain. The Kepler sketch offers a direct, dated anchor point that improves the timeline of the Sun’s transition from regular cycles to a grand minimum.