Johannes Kepler, the Renaissance astronomer known for his laws of planetary motion, made sunspot observations in 1607 that are now shedding new light on the sun’s activity cycles. Kepler’s drawings, accompanied by Latin descriptions, are the oldest datable sunspot records made using instrumental observation and projection. Kepler used a camera obscura method, projecting the sun’s image onto a sheet of paper through a small hole in a wall.

He initially thought he was witnessing a planet transiting the sun but later clarified that it was a group of sunspots. A new study by researchers from Japan’s Nagoya University suggests that Kepler’s drawings show the end of a solar cycle. This finding is significant because it predates the era when telescopes were widely used for such observations.

The study’s lead author, Hisashi Hayakawa, an assistant professor and solar scientist at Nagoya University, emphasized the importance of Kepler’s work. “This is the oldest sunspot sketch ever made with an instrumental observation and a projection,” Hayakawa said. The researchers analyzed Kepler’s drawings to determine the latitude of the observed sunspots.

Kepler’s instrumental sunspot observations

They found that the sunspot group was at a lower latitude than previously believed, indicating a transition between solar cycles. The study’s findings suggest that the sunspot group observed by Kepler was at the end of one solar cycle rather than the beginning of another.

The researchers place this transition zone between 1607 and 1610. The period between 1645 and 1715, known as the Maunder Minimum, was characterized by fewer sunspots and colder temperatures on Earth. While some researchers have theorized that the Maunder Minimum arose from irregular solar cycles, Kepler’s records indicate a regular duration for the cycle he observed.

However, the study’s findings are not without controversy. Previous studies using tree ring cycles have shown conflicting results, with some indicating regular solar cycles and others suggesting abnormalities. Hayakawa stressed the need for further research, saying, “It is extremely important to check these reconstructions with independent – preferably observational – records.”

The study’s findings were published in The Astrophysical Journal on July 25.

The researchers believe that Kepler’s early sunspot records could help resolve controversies regarding the duration of solar cycles in the early 17th century.