Solar activity refers to the state of the sun’s magnetic field and associated phenomena: sunspots, flares, solar wind and coronal ejections. During periods of minimal solar activity, such events are often uncommon and weak. During solar maximum, they’re at their strongest and most frequent. Magnetic field fluctuations on the sun can happen on drastically different timescales, ranging from seconds all the way to billions of years. When astronomers speak of a “slowdown” or a period of quiescence in the sun’s activity, it doesn’t mean the sun will stop shining, but that there’s a slowdown in activity.
The sun has one particular rhythm, lasting approximately 11 years, in which its polar magnetic field flips polarity. Sunspots serve as an indicator of this change. Indeed, it’s often known as “the sunspot cycle.”
Although sunspots themselves were first observed in detail by Galileo, Christoph Scheiner and others from 1609 onwards, according to the British Library, the cyclical nature of their appearance and disappearance was first noted in 1775 by Danish astronomer Christian Horrebow. It was then rediscovered in 1843 by Heinrich Schwabe. In 1848, Swiss astronomer Rudolf Wolf used Schwabe and others’ results, as well as performing his own observations, to calculate the 11-year cycle and a mathematical method to count the number of sunspots. This so-called “Wolf number” remains in use today, according to the National Oceanic and Atmospheric Administration (NOAA).
Many other astronomers at the time either independently observed this cycle or were inspired by the results of others. Wolf’s own calculation of the 1755 to 1766 sunspot cycle was labeled as the first, and each sunspot cycle since then has been progressively numbered as such. We are now in Cycle 25.
But sometimes the spots don’t appear at all. This was the case for 80 days of the first six months of the current solar cycle, which started in December 2019. It was greater still for the same period in Cycle 24, where there were 281 spot-free days. The period from 1645 to 1715 saw a near-total crash in sunspot numbers, where they could literally be counted on two hands.
Wolf struggled to piece together solar cycles before the mid-1700s because of this dearth of information, but it didn’t mean sunspots weren’t being observed. Many distinguished astronomers of the age, such as Giovanni Cassini, continued to make observations. This 70-year solar lull was later noted by German astronomer Gustav Spörer, which then later inspired the British-Irish husband and wife team Edward and Annie Maunder. The period has since been named the Maunder minimum, according to Encyclopedia Britannica.
