Corrected records of sunspot observations since 1610 demonstrate that the Sun's activity did not rise rapidly once the deep Maunder Minimum period of low activity reached its nadir in the late 17th century. During the Maunder Minimum—extending from about the middle of the 17th century to the beginning of the 18th century—sunspots were extremely rare and solar activity was unusually low. Solar activity slowly increased as the Maunder Minimum gradually came to an end.

During the Maunder Minimum, in most years, there were fewer than 50 individual sunspots and the annual mean Wolf sunspot number was below 20. But by contrast, during the Solar Cycle 24 (2008-2020), the average sunspot number was more than 75 and every year had more than 100 spots. Sunspot numbers have never reached such high levels in any of the 24 solar cycles since 1755, when extensive daily sunspot observations began.

"During the Maunder Minimum, the Sun was exceptionally calm," says lead author Dr. Sami Solanki, director at the Max Planck Institute for Solar System Research in Germany. "The corrected data show that the Maunder Minimum did not finish up abruptly, as often assumed, but ended with the gradual rise in sunspot numbers that became the precursor to the maximum of Solar Cycle 14 around 1780."

In particular, the new data show that a sharp peak in sunspot numbers around 1730 was not as great as previously thought, and the rise to the maximum of Solar Cycle 12 in the middle of the 18th century was more steady and gradual. Sunspots are dark patches on the Sun's surface that appear as temporary phenomena with a typical lifetime of a few days to several months. They consist of regions of intense magnetic field that suppress convection and consequently have a lower effective temperature than their surroundings, making them darker than the rest of the solar photosphere.

The research team carefully scrutinized the historic sunspot records that cover the earliest four centuries of daily observations. They cross-calibrated the original drawings of sunspots with the observations made nowadays from atop the 3,600-meter-high solar observatory at Izaña on Tenerife, Canary Islands, and with records of sunspot observations made at the Paris Observatory since the 19th century.

One unexpected difficulty for the researchers was the effect of the "telescope bias." "Before good telescopes were available at observatories, naked eye observers counted small clusters of sunspots as just one object, while larger clusters were counted as several entities. This means that over the centuries, the smallest sunspot clusters were undercounted," explains Solanki. "We had to correct for this, and this required analyzing the drawings, which was fascinating detective work."

The Sun is the main energy source for life on Earth, and significant variations of solar activity could affect the climate of the Earth. The corrected data will therefore also be valuable for further studies of the interplay of the Sun with Earth's climate.