Years before the first telescope was invented, sky watchers turned their rooms into pinhole cameras and took pencil and paper to draw the Sun and the small dark dots that moved across its surface day after day. The sunspot drawings date back more than 2,000 years and were made by astronomers in ancient china and, many centuries later, to Western scientists such as Galileo and Kepler.
Now, science historians around the world have come together to compile and digitize 400-year-old drawings of sunspots in hopes of illuminating past solar activity and informing our current understanding. solar physicist Andrés Muñoz Jaramillo used this digitized collection of sunspot observations to develop a collection of software tools to analyze solar cycles and reconstruct missing gaps.
“When we think about how much our ability to observe the [solar] “The cycle has evolved over the last few decades; it’s incredible,” said Muñoz-Jaramillo, a senior research scientist at the Southwest Research Institute in Boulder, Colorado. “The quality, the resolution, the cadence, everything.”
Learning from the past
Solar cycles typically last 11 years, but Muñoz-Jaramillo said the best instruments for observing the Sun, such as the Parker solar probe and the Solar Dynamics ObservatoryThey have been around for only about 2 decades. To understand solar variability dating back centuries, researchers must turn to techniques from the past.
“Whenever we’re faced with long-term variability, we can’t afford to wait 100 years for better data,” Muñoz-Jaramillo said.
Before the invention of photography, astronomers pointed a solar telescope at the Sun and used the eyepiece to project the image onto a paper-covered surface. They would draw the sunspots they observed that day and indicate the time and date. Over time, the spots appeared to move across the page and grow, shrink or change shape. some of these solar activity records They have survived to this day, often gathering dust in abandoned corners of archives.
Historians have been diligently collecting and digitizing centuries of drawings and creating detailed records of the position and size of the blots over time. Researchers are now using these records to study the Sun’s long-term variability.
“A lot of this work is done by our historian friends. They’re like detectives.”
“A lot of this work is done by our historian friends. They are like detectives,” Muñoz-Jaramillo said. “The real heroes are those who went from the archives to the basements and traveled all over the world and talked to people, convinced them to let them in and allowed them to take photographs.”
But data from hundreds of years is difficult to handle. So Muñoz-Jaramillo and his colleagues developed a computational framework to support the efforts of solar cycle researchers around the world. This collection of software tools uses Bayesian statistics to fill in gaps where sunspot data may not be available.
“Now you can make these statements probabilistically about what happened in these historical periods,” Muñoz-Jaramillo said.
The researchers used this new framework to learn more about the Maunder Minimum, a period in the 15th century when the Sun was less active and very few sunspots were observed: a few dozen compared to the tens of thousands typically observed. With so few data points, any additional information can help scientists better understand solar activity at that time, Muñoz-Jaramillo said. They also examined another period of slow activity in the late 16th century called Dalton Minimum and compared recent solar activity with that of previous centuries.
Using this framework, they learned that the Maunder and Dalton minima could have been preceded by other cycles with deep minima in solar activity widely separated in time. Some heliophysicists speculate that observations of complete solar cycles may be missing, Muñoz-Jaramillo said.
Muñoz Jaramillo and his colleagues presented these results on December 16 at the AGU 2025 Annual Meeting in New Orleans.
Detecting the evolution of the sun
Solar cycle researchers often observe cycles with what are known as butterfly diagrams, graphs of the time and latitude of sunspots. These charts can be used to understand the Sun’s long-term variability by comparing historical and modern data and observing parallels between them. Researchers can reconstruct past solar cycles using this new computational framework and analyze them using butterfly diagrams to better understand how the Sun has changed in recent centuries.
“It’s a service to the community. We put all these things together to make it easier for any modern scientist to work with.”
“This study is very innovative because, until now, reconstructions of past solar activity have been based solely on sunspot counts.” José Manuel Vaquero Martínez.a physics professor at the University of Extremadura who was not involved in the study said in an email. “In contrast, this approach incorporates not only the number of sunspots but also their positions. In other words, it leverages our understanding of how active solar regions (in this case, sunspots) evolve to reconstruct past solar activity.”
The team hopes their work will allow researchers to tap into the trove of historical data more easily than before, Muñoz-Jaramillo said. “It’s a service to the community. We put all these things together to make it easier for any modern scientist to work with.”
—Daniella García Almeida, science writer
Citation: García Almeida, D. (2025), Drawings of sunspots illuminate 400 years of solar activity, Éos, 106, https://doi.org/10.1029/2025EO250477. Published on December 17, 2025.
Text © 2025. The authors. CC BY-NC-ND 3.0
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