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Johannes Kepler thought he sketched Mercury orbiting across the sun. What he actually captured has solved a solar mystery
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German astronomer Johannes Kepler made sketches of sunspots in 1607 from his observations of the sun’s surface — and centuries later, the pioneering drawings are helping scientists solve a solar mystery.

Even though everything in the solar system revolves around the sun, scientists have yet to unlock many of the star’s secrets.
However, studying the variability of the sun over time, including the solar cycle, could answer some of the most longstanding questions about the fiery orb and how it changes.
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Some of those questions revolve around solar activity in the 17th century, which was a pivotal time for studying the sun.

Astronomers observed sunspots with telescopes for the first time in 1610. At the same time, the sun was making an unusual transition into an extended period of weakened activity. And Kepler’s long disregarded sketches, overlooked because they were drawings rather than telescopic observations, could provide crucial historical insights.

A new study that recreates the circumstances during which Kepler made his drawings appeared on July 25 in The Astrophysical Journal Letters.

“Kepler contributed many historical benchmarks in astronomy and physics in the 17th century, leaving his legacy even in the space age,” said lead study author Hisashi Hayakawa, assistant professor at Nagoya University’s Institute for Space-Earth Environmental Research, in a statement.

“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.”

The sun’s tumultuous activity
The sun experiences an 11-year cycle of waxing and waning activity, known as the solar cycle. Currently, scientists believe that the sun is reaching or nearing solar maximum, the annual peak of its activity for the current solar cycle, called Solar Cycle 25.

Solar maximum is typically associated with an increase in the number of sunspots visible on the sun’s surface. These dark regions, some of which can reach the size of Earth or larger, are driven by the sun’s strong and constantly shifting magnetic fields.

Today, scientists track solar activity using data from ground and space-based observatories, magnetic maps of the solar surface, and ultraviolet observations of the sun’s outer atmosphere.