What the solar cycle is

The Sun’s magnetic activity rises and falls in a pattern called the solar cycle. It lasts about eleven years on average, though individual cycles have ranged from roughly nine to fourteen years. The most visible sign is the number of sunspots, dark patches on the Sun’s surface that are cooler than their surroundings.

At solar minimum, the Sun can go days without a visible sunspot. At solar maximum, many groups of spots appear, and eruptions such as flares and coronal mass ejections become more frequent.

How the cycle was discovered

Astronomers have recorded sunspots since the invention of the telescope in the early seventeenth century. In 1843, the German amateur astronomer Samuel Heinrich Schwabe announced a regular pattern after about seventeen years of observations.

Daily sunspot counts have been compiled since 1749, and numbered cycles follow from that record. NASA and NOAA marked the start of Solar Cycle 25 in December 2019, and in 2024 they announced that the Sun had reached the maximum period of that cycle.

Space weather

Eruptions from the Sun can disturb Earth’s magnetic field in what is called a geomagnetic storm. These storms can produce auroras at lower latitudes than usual, and they can also interfere with satellites, radio communication and power grids.

The best-known historical event is the Carrington Event of September 1859, when telegraph systems failed in several places and auroras were seen far from the poles. Agencies now monitor the Sun continuously to give warnings.

Weather and climate

A more difficult question is how the cycle affects Earth’s weather and climate. The total energy the Sun delivers changes by only about a tenth of a percent over a cycle, and researchers continue to study possible indirect effects, for example in the upper atmosphere.

Scientists generally distinguish the Sun’s well-documented effects on space weather from claims that it controls specific storms or extreme weather events on the ground. The second kind of claim needs evidence that can be tested against records.

How to read solar claims

When you see a claim linking solar activity to events on Earth, look for the data behind it. Ask how the timing was measured, whether the pattern holds across many cycles and whether other explanations were considered.

Public sources such as NOAA’s Space Weather Prediction Center and the sunspot records of the Royal Observatory of Belgium make the underlying numbers easy to check, which is a good starting point for any independent investigation.