This is a collaboration with the story Biography.com.
Few things seem more immutable than the Sun—after all, it’s been fusing hydrogen for five billion years, and it will continue to do so for another five billion years. But despite our star’s magnificent existence, the Sun’s magnetic field will experience several solar cycles in just one human lifetime.
These 11-year-long phases wax and wane between solar minimum and maximum, which are respectively characterized by more or less solar activity (sunspots, flares, magnetic fields, etc.). Currently, we are living Through the 25th 11-year cycle After astronomers began closely tracking the Sun’s magnetic activity in 1755.
But at the heart of these apparently well-ordered solar cycles lies a mystery.
From 1645 to 1715, the Sun experienced a sustained period of depressed solar activity known as the Grand Solar Minimum, or Mandar Minimum; The phenomenon is named after the English astronomer Edward Walter Mander who discovered it. To fully understand this grand minimum, astronomers need data from the unpredictable solar quiet east. But since the first telescope observations of the Sun came only a few decades before Maunder minimum, data is hard to come by.
Good thing, then, that we have the German astronomer Johannes Kepler.
Kepler is best known for his laws of planetary motion (as well as being the namesake of one of NASA’s most important space telescopes), but in 2024, scientists at Nagoya University in Japan, analyzing observations Kepler made of sunspots in 1607 using the Camera Obscura—announced that they believe the data can be reinterpreted. Mander Min.
And while that reinterpretation is certainly necessary, Kepler initially thought he was seeing a transit of Mercury. The results of the study were published in 2015 Astrophysical Journal Letters.
“Because this record is not a telescopic observation, it has only been discussed in the context of the history of science and was not used for quantitative analysis of the solar cycle in the 17th century,” Hisashi Hayakawa of Nagoya University, lead-author of the study, said in a press release. “But this is the earliest sunspot sketch made with instrument observations and projections.”
To correctly interpret Kepler’s original findings, Hayakawa and his team needed to narrow down the time the observations were taken and reconstruct the position of the features on the Sun’s surface (called the heliographic inclination). Previously, astronomers have relied on tree-ring observations—when the Sun is particularly active, the solar wind and solar magnetic field shield Earth well from galactic cosmic rays (which are inscribed in tree rings as carbon-14). Higher solar activity means lower carbon-14 (and vice versa), and you can see those changing levels in tree rings.
But depending on only Tree rings come with some problems in understanding solar cycles, as three separate observations place these cycles (in this case Solar Cycles -13 and -14) into extremely short, normal, and extremely long categories. Here comes Kepler’s observations, and scientists discovered that this 417-year-old observation probably took place in the tail-end solar cycle-13 rather than the beginning of -14.
Later telescopic observations also detail how Kepler’s diagram indicates a general transition from the previous solar cycle, and the team may now have shrunk significantly when that transition occurred—between 1607 and 1610. The end result? During this time, the Sun exhibited a typical solar cycle.
“By placing Kepler’s findings in the larger solar activity reconstruction, scientists have gained an important context for explaining the changes in solar behavior during this pivotal period from the regular solar cycle to the grand solar minimum,” Hayakawa said in a press release. “Kepler’s sunspot record predates existing telescopic sunspot records from 1610 by several years. His sunspot sketches serve as a testament to his scientific skill and perseverance in the face of technological obstacles.”
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