How Sunspots Work

The History of Sunspots

Astronomers in ancient China noticed sunspots several thousand years ago. The I-Ching or "Book of Changes," which dates back to the 12th century B.C., mentions a "Ri Zhong Jian Mei," which means "a star was seen within the sun" in English. The first written record of a sunspot sighting dates to 28 B.C., when it was noted that "the sun was yellow at its rising and a black vapor as large as a coin was observed at its center." On the other side of the world, the Aztecs, who ruled Mexico before the Spanish arrived in the 1500s, also paid a lot of attention to the sun. As we discussed, their creation myth featured a sun god with a pockmarked face.

In Europe, people had a harder time accepting the existence of sunspots. That was because everyone -- including the Catholic Church -- accepted Greek philosopher Aristotle's idea that the heavens were perfect and unchanging. Instead, when a large sunspot appeared for eight days in A.D. 807, they dismissed the phenomenon as the passage of the planet Mercury across the sun.

However, after the telescope was developed in the early 1600s, the Italian astronomer Galileo and others clearly saw that the sun had dark spots. Astronomer and Catholic priest Christoph Scheiner tried to come up with an explanation that didn't contradict Church teachings; he argued that the spots actually were undiscovered planets that orbited very close to the sun and were visible only when the planets were in front of the sun. Despite Scheiner's attempts, Galileo correctly figured out that sunspots were part of the sun itself by closely studying the movement of sunspots over time. By the mid-1700s, European astronomers were recording and compiling their observations of sunspots on a daily basis.

As scientists accumulated more and more data, they began to notice that sunspot activity developed a pattern. In 1843, astronomer S.H. Schwabe was the first to describe the 11-year sunspot cycle.

Since then, scientists used have used an array of tools -- including giant solar telescopes that were specially cooled to observe the sun's light without being distorted by its heat -- to learn more about the physics of sunspots. Astronomer George Ellery Hale discovered sunspots' magnetic nature and used that discovery to prove the existence of a large magnetic field in the sun's interior. More recently, astronomers have discovered starspots -- sunspots on other stars. One giant star, HD 12545, bears a spot 10,000 times larger than the biggest spots observed on the sun.

Sunspot Cycle FAQ

What causes a sunspot?

Sunspots occur because the sun is a ball of continually circulating hot gases that doesn't move in one piece. The interior and the exterior of the sun rotate separately; the outside rotates more quickly at the equator than at the solar north and south poles. Over time, that uneven movement twists and distorts the sun's main magnetic field. The bunched up spots, actually twists in the magnetic field lines, have so much magnetic power that they push back the hot gases beneath them and prevent the heat from rising directly to the surface, which creates sunspots.

What is the frequency of the sunspot cycle?

Roughly every 11 years, the number of sunspots increases from nearly zero to more than 100 and then decreases to near zero again as a new cycle starts. This pattern is called the sunspot cycle.

What causes the sunspot cycle?

The sunspot cycle occurs because the sun contains a sort of conveyor belt that circulates a hot, electrified gas called plasma between the sun's equator and its poles and then back again over a period of years.

Do sunspots affect climate?

It's unclear if there's a link between solar weather and changes in Earth's climate because our planet's climate is influenced by so many other factors, from volcanic eruptions to man-made emissions of greenhouse gases.

How long does a sunspot last?

The sunspot cycle lasts about 11 years.

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