Solar activity rises and falls on a cycle of roughly eleven years, and the measure used to track it is a count of dark spots visible on the solar surface.
Sunspots mark concentrated magnetic field
A sunspot appears dark because intense magnetic field there suppresses the convection that carries heat upward, leaving that patch cooler than its surroundings.
The same concentrated field stores energy, and its sudden reconfiguration is what produces flares and coronal mass ejections.
Counting spots therefore serves as a proxy for how much stored magnetic energy the Sun is carrying at a given time.
The count follows a defined convention
The standard number is not a raw tally. Groups of spots and individual spots are weighted together, because spots cluster within active regions.
The convention dates to the nineteenth century and has been maintained since, with adjustments applied so observations from different observers remain comparable.
Continuity is what gives the record its value, since a consistent method across many decades allows one cycle to be compared meaningfully with another.
Cycles differ in strength and length
The eleven-year figure is an average rather than a fixed period, and individual cycles have run noticeably shorter or longer than that.
Peak amplitude varies substantially too, with some cycles producing far more spots and far more eruptive activity than their neighbors.
Extended quiet stretches appear in the historical record, periods when spots were scarce for decades, which is one reason the long series is studied closely.
Latitude drift reveals the underlying magnetic cycle
Spots emerge at middle solar latitudes early in a cycle and appear progressively closer to the equator as it advances.
Plotted over time this drift forms a distinctive pattern, and it reflects the migration of the magnetic structures generating the spots.
The Sun's overall magnetic polarity reverses near each maximum, which means the full magnetic cycle is about twice the length of the spot cycle.
Activity level shapes practical forecasting
Near solar maximum, eruptions are more frequent, so geomagnetic storms, radio disruptions and aurora sightings all become more common.
Increased ultraviolet output also heats and expands the upper atmosphere, raising drag on satellites in low orbit and shortening their working lifetimes.
Forecast centers use the current position within the cycle to set baseline expectations, then rely on direct solar observation for individual events.
Cycle predictions themselves remain difficult, since the strength of an upcoming maximum has to be inferred from magnetic conditions late in the cycle preceding it.