What is the solar cycle?
The solar cycle is the roughly 11-year rise and fall of the Sun's magnetic activity. Individual cycles have lasted from about nine to fourteen years, and the most visible marker is the number of sunspots, dark patches on the Sun's surface that are cooler than the surrounding gas. At solar minimum the Sun can go days with no spots, while at solar maximum many sunspot groups appear and flares and coronal mass ejections become more frequent.
The cycle is driven by the Sun's magnetic field. Roughly every 11 years the Sun's magnetic poles reverse, so a complete magnetic cycle, called the Hale cycle after the astronomer George Ellery Hale, spans about 22 years. Sunspot counts are the long-standing way to track that underlying behavior.
How was the cycle discovered?
Telescopic observers began recording sunspots in the early seventeenth century. In 1843 the German amateur astronomer Samuel Heinrich Schwabe announced that sunspot numbers rise and fall in a regular pattern, after about seventeen years of observation. Later astronomers assembled earlier records into a continuous sequence and numbered the cycles. By the standard numbering, Cycle 1 began in 1755.
The official sunspot series is maintained by the Royal Observatory of Belgium's SILSO center, which reaches back to the mid-1700s and has been revised since the 2015 recalibration. That revision is a reminder that even basic solar data is a constructed record, built from many observers and instruments, and therefore open to improvement.
Where are we in Solar Cycle 25?
NASA and NOAA place the start of Solar Cycle 25 at the minimum of December 2019. In 2019 an international prediction panel convened by NOAA, NASA and the International Space Environment Service expected a fairly weak cycle peaking around July 2025 with a maximum sunspot number near 115. Observed activity ran ahead of that forecast, and in 2023 NOAA's Space Weather Prediction Center revised its outlook to a peak between January and October 2024, with a maximum sunspot number between 137 and 173.
On October 15, 2024, NASA and NOAA announced that the Sun had reached the solar maximum period of the cycle. NOAA's Elsayed Talaat noted in the announcement that this did not mean the peak had been reached or passed. Scientists identify the actual peak only in hindsight, after months of smoothed data, and the maximum phase can last a year or more. For current numbers always check the live pages of NOAA's Space Weather Prediction Center and SILSO rather than a static article.
What is space weather, and what can it affect?
Space weather is the changing condition of the near-Earth environment caused by the Sun. A coronal mass ejection is a huge cloud of magnetized plasma, and when one arrives at Earth it can compress and disturb the planet's magnetic field in a geomagnetic storm. A solar flare is a burst of radiation that travels at the speed of light and can disrupt high-frequency radio on the sunlit side within minutes.
Documented effects include auroras at lower latitudes than usual, extra drag on satellites in low orbit, interference with GPS and radio, and, in extreme cases, stress on power grids. NOAA rates geomagnetic storms on a scale from G1 to G5. In May 2024 a G5 storm produced auroras seen far from the poles, and it was reported as the strongest storm in two decades. These effects are measured and are the core of what the field of space weather studies.
What was the Carrington Event?
On September 1, 1859, the British astronomer Richard Carrington observed an intense flare on the Sun, and within a day or so a severe geomagnetic storm hit Earth. Telegraph systems in North America and Europe failed or gave operators shocks, and auroras were reported at unusually low latitudes.
It remains the benchmark for an extreme storm in the modern record, and it is why agencies study the risk to technology that did not exist in 1859. Because the event predates modern instruments, estimates of its strength rely on historical observations and on magnetic records from the period, which scientists continue to reassess.
What was the Maunder Minimum?
From roughly 1645 to 1715 astronomers recorded very few sunspots, an interval named the Maunder Minimum after the astronomer Edward Maunder. Observers at the time, including those at the Paris Observatory, noted how rare spots had become, and records of auroras were also sparse.
The period partly overlaps with colder conditions in parts of Europe sometimes called the Little Ice Age. Researchers continue to debate how much, if any, of that cooling is due to reduced solar output, since volcanic eruptions and internal climate variability also played a role. The overlap in time is documented, but the strength of the causal link is not settled.
How much does the cycle change Earth's weather and climate?
The total energy the Sun delivers changes by only about a tenth of a percent over a cycle, according to satellite measurements since the late 1970s. Most of the variation is in ultraviolet light and the particle environment, which mainly affect the upper atmosphere. Researchers are studying whether those changes influence regional circulation at lower altitudes, and some modeling studies report modest effects, but they remain an active area of research.
The scientific consensus separates two kinds of claims. Effects of the Sun on space weather and technology are well documented. Claims that solar activity controls specific earthquakes, hurricanes or heat waves on the ground need statistical evidence that holds across many cycles, and that evidence has generally not been established. A pattern in a short window or a small number of events is not proof of a cause.
Why is the solar cycle hard to forecast?
The official forecasts show how uncertain it is. The 2019 panel expected a weak Cycle 25 peaking around July 2025 with a maximum sunspot number near 115. By 2023 NOAA had raised the expected maximum to between 137 and 173 and moved the peak window to between January and October 2024. Observed sunspot counts exceeded the original forecast for many consecutive months.
Part of the difficulty is that the Sun's interior dynamo cannot be observed directly, so models rely on indicators such as the strength of the polar magnetic field at the previous minimum and on patterns in earlier cycles. Those methods have worked better for some cycles than for others. For readers, the lesson is practical: treat any single prediction, including ones that claim a precise date for a storm, as a probability and not a schedule.
What can you actually watch for during solar maximum?
For most people the visible effect is aurora. Strong geomagnetic storms push the northern and southern lights toward the middle latitudes, and NOAA publishes short-term aurora forecasts and watch alerts. For anyone who relies on GPS, high-frequency radio or satellite services, the same agency issues watches and warnings for radio blackouts, solar radiation storms and geomagnetic storms using a scale from 1 to 5.
Everyday life continues as normal during nearly all of these events. The worst documented disruptions have been to specialized systems, and agencies and utilities have increased monitoring since the 1989 geomagnetic storm that interrupted power in Quebec. Following the data from public sources is both the most practical and the most reliable way to stay informed.
How should you test a claim linking the Sun to events on Earth?
Look for the dataset first. Ask which sunspot or flare measure was used, how the event on Earth was defined, and what the time window was. Check whether the pattern holds across several cycles rather than one, and whether the author compared it with random dates to see how often a match would happen by chance.
Then check whether competing explanations were considered, and whether the source is a measurement, a model or an opinion. Sunspot data from SILSO, forecasts from NOAA and mission results from NASA are public, so you can reproduce a simple comparison yourself. That habit of separating measured data from interpretation is what the book The Code of the Sun applies to its subject.