This story is a collaboration with Biography.com.
Few things seem more constant than the Sun—after all, it has been fusing hydrogen for 5 billion years and will continue to do so for about 5 billion more. But despite our star’s enormous existence, the Sun’s magnetic field will go through many solar cycles in just one lifetime.
These 11-year periods ebb and flow between solar minimum and maximum, characterized by more or less solar activity (sunspots, halos, magnetic fields, etc.), respectively. We are living now to the 25th 11-year cycle since astronomers began closely monitoring the Sun’s magnetic activity in 1755.
But there’s a mystery lurking at the heart of these seemingly neat solar cycles.
From 1645 to 1715, the Sun experienced a prolonged period of reduced solar activity known as the great solar minimum, or Maunder minimum; The phenomenon is named after the British astronomer who discovered it, Edward Walter Maunder. To fully understand this grand minimum, astronomers need data that preceded the unexpected solar lull. But with the first telescopic observations of the Sun coming only a few decades before the Maunder minimum, data were difficult to come by.
It’s a good thing we have German astronomer Johannes Kepler.
Kepler is best known for its laws of planetary motion (as well as being the namesake of a very important NASA space telescope), but in 2024, scientists from Nagoya University in Japan – analyzing observations Kepler made of sunspots in 1607 using a camera obscura – announced that they believed this reinterpreted piece of data could help astronomers unravel the mysteries about the Maunder minimum.
And that reinterpretation was certainly necessary, since Kepler initially thought he was witnessing the transit of Mercury. The research results are published above Astrophysical Journal Letters.
“Since this record is not a telescopic observation, it is only discussed in the context of the history of science and has not been used for quantitative analysis of solar cycles in the 17th century,” Hisashi Hayakawa, the study’s lead author, said in a press release. “But this is the oldest sunspot sketch ever made with an observer and a projector.”
To properly interpret Kepler’s initial findings, Hayakawa and his team needed to narrow down when the observations were made and reconstruct the locations of features on the Sun’s surface (what is known as heliopause). In the past, astronomers have relied on tree-ring observations – when the Sun is particularly active, the solar wind and solar magnetic field better protect Earth from galactic cosmic rays (which are etched into tree rings as carbon-14). High solar activity means lower carbon-14 (and vice versa), and you can see those changing levels in tree rings.
But rely on it just one tree rings to understand solar cycles comes with some problems, as three separate observations classify these cycles (in this case Solar Cycles -13 and -14) as extremely short, normal, and even extremely long. This is where Kepler’s observation comes in, and scientists discovered that this 417-year-old observation likely occurred at the end of Solar Cycle -13, instead of the beginning -14.
Later telescopic observations also detailed how Kepler’s drawings were able to show a typical transition from the previous solar cycle, and the team could now significantly narrow down when that transition took place – from 1607 to 1610. The end result? During this time, the Sun exhibits a typical solar cycle.
“By placing Kepler’s findings within a broader reconstruction of solar activity, scientists gain important context for explaining changes in the sun’s behavior during the key period marking the transition from the regular solar cycle to the solar minimum,” Hayakawa said in a press release. “Kepler’s sunspot records predate existing telescopic sunspot records from 1610. His sunspot sketches are a testament to his scientific acumen and perseverance in the face of technological limitations.”
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