Even if the Sun always looks the same to us, sunspots—whose sizes vary—were already identified back in the 18th century. The effort to understand what causes these changes led to a fascinating journey into the Sun’s activity cycles and the influence of the planets on it.
In times of uncertainty, we often comfort ourselves by saying, "the sun will come up tomorrow." And it's true—whether hidden behind clouds or blazing in a clear sky, the sun rises every single day. But is it really the same Sun we see each morning? That question has intrigued scientists for many years.
Before exploring the short-term variations in solar activity, it's worth noting that the Sun is about 4.6 billion years old and is expected to last for another 5 billion years. On a human timescale—which is much shorter than the Sun's lifespan—this suggests relative stability. However, as early as the 18th century, Danish astronomer Christian Horrebow identified cyclical changes in the Sun’s activity. By tracking sunspots over 15 years, Horrebow noticed that their number and size followed a regular pattern. These sunspots, which appear as dark patches on the Sun’s surface roughly the size of Earth, are regions of intense magnetic activity and lower temperatures compared to the surrounding solar surface.
In the 19th century, amateur astronomer Heinrich Schwabe discovered a repeating pattern in sunspot activity lasting about ten years. This became known as the Schwabe Cycle. Modern observations have confirmed that the average solar cycle spans approximately 11.07 years.
The large sunspot in the lower-left corner is about 11 times the size of Earth. Sunspots photographed in 2012. Source: NASA Goddard Space Flight Center, Greenbelt, MD, USA
Back to the Future
To explain the cause of the Schwabe Cycle, we need to fast-forward to the 20th century. American astrophysicist George Ellery Hale showed that sunspots are areas of intense magnetic activity, and that the Sun's magnetic poles reverse approximately every 11 years—matching the length of the Schwabe Cycle.
Let’s briefly compare the Sun’s magnetic cycle to the cycles we are familiar with on Earth. Earth rotates on its axis once every 24 hours, so each longitude faces the Sun once per rotation. In contrast, Earth’s orbit around the Sun takes a year. In both cases, Earth's cycles are determined by the Sun. In contrast, the Sun’s own cycle is driven solely by internal factors. But is that the whole story?
Tides, Ebbs, and Everything In Between
As early as the 19th century, researchers began exploring whether planets in the solar system influence the Schwabe Cycle. To understand how planets—whose mass is much smaller than the Sun's—might have an effect, let’s turn to a more familiar example in our neighborhood: Earth and the Moon. The Moon’s mass is about 1.2% of Earth's, and because of their relative proximity, it follows a gravitational orbit around Earth. Despite its smaller size, the Moon’s gravitational pull on Earth is significant—for instance, it is responsible for ocean tides.
So, could the planets exert a similar influence on the Sun’s activity? Indeed, studies have shown that the tidal forces exerted by Earth, Jupiter, and Venus on the Sun operate in an 11-year cycle, much like the Schwabe Cycle.
So, what governs the Schwabe Cycle: the Sun’s magnetic reversal or the planetary influences of the surrounding planets? A recent study suggests a compromise. According to this research, it's a combination of both: while the Sun’s magnetic reversal follows a natural cycle of about 11 years, the gravitational tugs from Earth, Jupiter, and Venus—operating in a cycle of 11.07 years—may act as a “clock,” helping to stabilize the rhythm of that cycle.
To support their claim, the researchers showed that the planets also influence other solar activity cycles, from the Rieger-type cycle (ranging from 100 to 300 days) to the Bray-Hallstatt cycle, which spans more than two thousand years. Therefore, to accurately calculate the Sun’s activity cycle, it’s essential to consider both internal solar dynamics and external gravitational influences from the orbiting planets—highlighting that these planets may play a more significant role in solar behavior than previously thought.