Renaissance astronomer Johannes Kepler’s “half-forgotten” sunspot drawings tell us more about how the Sun’s activity cycle works.
Kepler (1571-1630), born in what we now call Germany, is best known for astronomy to formulate the laws of planetary motion. His various interests, however, included the observation the sun. Drawings he made of a group of sunspots in 1607, a new study reveals, show the “tail of the solar cycle“with instruments prior to the appearance of the telescope at the beginning of the 17th century.
“The group’s findings… offer a key to resolving the controversy over the length of solar cycles in the early 17th century,” wrote Japan’s Nagoya University. in a report.
Known as the Maunder Minimum, this period (between 1645 and 1715) was supposed to be an era when there were fewer sunspots than usual, leading to periods on Earth that were colder than the norm during the day.
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Sunspots are cooler regions on the Sun’s surface that can generate eruptive disturbances, such as solar flares and coronal mass ejections. (Image credit: NASA/SDO)
Because Kepler didn’t have a telescope, he instead examined the Sun using a camera obscura. This method used a “small hole in a wall to project the image of the Sun onto a sheet of paper,” the statement said. Kepler initially thought he was witnessing a transit of Mercury across the sun, but it was later clarified that it was a sunspot group.
“This is the oldest sunspot sketch ever made from instrumental observation and projection,” Hisashi Hayakawa, an assistant professor and solar scientist at Nagoya and lead author of the study, said in a statement. He added that the significance of Kepler’s solar drawings had been overlooked over time: “They were only discussed in the context of the history of science and were not used for quantitative analyses of solar cycles.”
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The researchers’ analysis consisted of tracking the evolution of sunspots over the course of a solar cycle: “Their occurrence, frequency, and latitudinal distribution appear in cycles that affect solar radiation and space weather,” the statement said. So they worked to determine at what latitude Kepler’s sunspots were observed.
Johannes Kepler wearing a collar and black shirt and holding a mathematical compass, in a painting
Johannes Kepler (1571-1630), German mathematician, astronomer and astrologer, oil on panel, anonymous, 1610. (Photo credit: History Images/Universal Images Group via Getty Images)
The researchers say their analysis of Kepler’s drawings revealed four main things.
First, the sunspot group was at a lower latitude than previously thought (after taking into account the angle of the sun’s position relative to its location); related to this, future telescope findings showed sunspots at higher latitudes, suggesting “a typical transition” between cycles.
The change in latitude suggests that the sunspot group was at the end of one solar cycle instead of the beginning of another, based on the latitude of the sunspots, which formulated the third discovery.
Finally, Kepler’s observations could show the transition zone between solar cycles, which the study places between 1607 and 1610.
While some researchers had previously theorized that the Maunder Minimum resulted from irregular solar cycles other than the usual 11 years, Kepler’s records showed “a regular length” for the solar cycle he observed in 1607, the study adds.
But the discovery is not without controversy: tree ring cycles from previous studies have suggested normal solar cycles in some works, and abnormal solar cycles in others.
Hayakawa called for further investigations into the matter. “It is extremely important to verify these [tree rings] “reconstructions with independent recordings – preferably observational,” he said.
The new study was published in The Astrophysical Journal July 25.
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