How Sunspots Work

By: Patrick J. Kiger

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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More Great Links


  • Boozer,Christopher. "Genesis Search for Origins Fact Sheet," Undated. (March 19, 2009)
  • The Columbia Encyclopedia. "Sunspots," Columbia University Press, 2007.
  • Department of Physics and Astronomy, Georgia State University. "Sunspots and Solar Storms," 2005.
  • (March 19, 2009)
  • Encyclopedia Britannica Online. "Telescope," 2009. (March 19, 2009)
  • European Space Agency. "Frequently Asked Questions." 2009. (March 25, 2009)
  • European Space Agency. "What Are Solar Flares?" 2009. (March 19, 2009)
  • Exploratorium. "Sunspots: Modern Research," 1998. (March 19, 2009)
  • Exploratorium. "Sunspots: History." 1998. (March 19, 2009)
  • Galileo Project. "Sunspots," 1995. (March 19, 2009)
  • Imberi, Jonathan and Sara. "Radio Wave Propagation," 2009. (March 25, 2009)
  • Koppes, Steve. "No Escaping Charged Particles During Solar Maximum," Daily University Science News, August 7, 2001. (March 19, 2009)
  • Lang, Kenneth R. "NASA's Cosmos: Inside the Sun," 2003. (March 19 2009)
  • Laster, Clay. "The Beginner's Handbook of Amateur Radio," McGraw-Hill Professional, 2000.
  • Leinbach, H. "Equatorial Acceleration of Sunspots," Popular Astronomy, Vol. 57, p. 427. 1949. (March 19, 2009)
  • MacRobert, Alan M. "Viewing the Sun Safely," Sky & Telescope, 2009. (March 19 2009)
  • McKee, Maggie. "Maverick Sunspot Heralds New Solar Cycle," New Scientist, January 2008. (March 19, 2009)
  • National Aeronautics and Space Administration (NASA). "The Big Questions," 2007. (March 19, 2009)
  • NASA. "Chilly Temperatures During the Maunder Minimum," November 15, 2006. (March 19, 2009)
  • NASA. "Earth-Directed Sunspot Now Size of 20 Earths," July 22, 2004.
  • (March 19, 2009)
  • NASA. "Solar Cycle 24 Begins." January 10, 2008. (March 19, 2009)
  • NASA. "Solar Physics," 2009. (March 19, 2009)
  • NASA. "Solar Storm Warning," June 7, 2000. (March 19, 2009)
  • NASA. "The Sunspot Cycle." 2009. (March 19 2009)
  • NASA. "What's Wrong with the Sun?" July 11, 2008. (March 19 2009).
  • National Oceanic and Atmospheric Administration. "The Sun and Sunspots: Can an Increase or Decrease in Sunspot Activity Affect the Earth's Climate?" 2006. (March 19, 2009)
  • National Optical Astronomy Observatory. "McMath-Pierce Solar Telescope," 1999. (March 19, 2009)
  • National Solar Observatory at Kitt Peak. "Accomplishments of the McMath-Pierce Telescope," 2007. (March 19, 2009)
  • National Solar Observatory, "Mr. Sunspot: Sun-Earth Connection," (March 25, 2009).
  • Ridpath, Ian (editor). "A Dictionary of Astronomy." Oxford University Press, 1997.
  • Sanders, Robert. "Was 17th Century Solar Funk a Rarity?" UC Berkeley News, June 1, 2004. (March 25, 2009)
  • Seligman, Courtney. "The Sunspot Cycle." 2009. (March 19, 2009)
  • Space Environment Center. "Solar Maximum," 1999. (March 19, 2009)
  • Solar and Heliospheric Observatory, "About the SOHO Mission," 2007. (March 19, 2009)
  • Taylor, P.O. "Comparing the March 1989 Sunspot Group with Other Great Groups of the past," Journal of the American Association of Variable Star Observers. 18:1, p. 65-69, 1989. (March 25, 2009)
  • Ternullo, M. "After a Century with Maunder's Butterfly Diagram," Mem. S.A. It., Vol. 78, p. 596. 2007. (March 19, 2009)
  • University Corporation for Atmospheric Research, "Breakthrough Research to Improve Forecasts of Sunspot Cycle," May 31, 2004. (March 19, 2009)
  • University Corporation for Atmospheric Research, "Rotation of the Sun," 2005. (March 19, 2009)
  • University Corporation for Atmospheric Research, "Starspots," 2005. (March 19, 2009)
  • University Corporation for Atmospheric Research, "The Sunspot Cycle," 2005. (March 19, 2009)
  • University Corporation for Atmospheric Research, "Sunspots and Magnetic Fields," 2005. (March 19, 2009)
  • Vaquero, J.M. "Historical Sunspot Observations: A Review," No date. (March 19, 2009)
  • Weinstock, Maia. "Largest Sunspot in Nine Years Found on Sun,", September 21, 2000. (March 19, 2009)
  • Xu Zhentao, "Solar Observations in Ancient China and Solar Variability," Philosophical Transactions of the Royal Society London, Vol. 300, p 513-515, 1990. (March 19, 2009)