SUNSPOTS Dark spots, some as large as 50,000 miles in diameter, move across the surface of the sun, contracting and expanding as they go. These strange and powerful phenomena are known as sunspots.

Galileo and the Advent of the Telescope The earliest astronomers had to rely on their eyes to observe the sun. Given the proper conditions (such as fog or haze or viewing the sun at sunset), it is possible to observe sunspots with the naked eye. Viewing the sun in this way presented a problem, since looking directly at the bright sun is not only hard to do, but very dangerous. You should never look directly at the sun. The sun's radiation contains not only visible light, but powerful ultraviolet radiation as well, which can severely burn your eyes and permanently damage your eyesight. The invention of the telescope by Dutch craftsmen in about 1608 changed astronomy forever. Suddenly, European astronomers could peer into space, seeing previously unimagined details on known objects like the moon, sun, and planets, and discovering planets and stars never before visible.

There is some debate over which European was first to discover sunspots. The credit is usually shared by Johann Goldsmid (known as Johannes Fabricius) of Holland, Galileo Galilei of Italy, Christopher Schiener of Germany, and Thomas Herriot of England, all of whom claimed to have discerned sunspots sometime in All four men observed sunspots through telescopes, and made drawings of the changing shapes by hand, watching the spots traverse the visible surface of the sun. These drawings were the first steps toward understanding sunspots

Galileo made a breakthrough. By observing the sun closely over a period of several weeks Galileo noticed the shape of the sunspots became foreshortened as they approached the edge of the visible sun. He realized that this would only happen if the spots were objects on the surface of the sun, and not if they were spherical planets or moons passing before the sun. So he concluded that the spots must be on the surface.

Safely Viewing Sunspots How did the early astronomers manage to look at the sun? One of the methods they figured out was projection. By using a telescope to project a smaller, dimmer image of the sun, they were able to study the sun at their leisure, with no risk to their eyes.

The safest practical way to see the sun is by lining up a telescope or binoculars on the sun, but instead of looking through the eyepiece, hold a sheet of white paper behind the eyepiece. The solar image is seen projected onto the paper, and you can safely look at it there.

The Sunspot Cycle In the last few decades, the forces behind sunspots are becoming better understood, but we've known for over a 150 years that sunspots appear in cycles. The average number of visible sunspots varies over time, increasing and decreasing on a regular cycle of between 9.5 to 11 years, on average about 10.8 years. An amateur astronomer, Heinrich Schwabe, was the first to note this cycle, in The part of the cycle with low sunspot activity is referred to as "solar minimum" while the portion of the cycle with high activity is known as "solar maximum." By studying the sun's magnetic field, modern astronomers have discovered that the cycle covers twenty-two years, with an eleven-year cycle of sunspots above the equator followed by an equal cycle below the equator. According to Fischer, "the overall magnetic field structure changes in a way that is very interesting: It turns out that the magnetic fields primarily point from west to east in the Northern Hemisphere (of the sun), and from west to east in the Southern Hemisphere. In the next eleven-year cycle, the fields are reversed. So the cycle is really twenty-two years."

Sunspots most often appear in the low latitudes near the solar equator, and they almost never appear below 5 or above 40 degrees north and south latitude. As the sunspot cycle progresses, the visible sunspots move gradually towards the equator.

Telescopes today operate in different wavelengths, so the fellows in Galileo's time had to just use the eye, and just use visible light. Today, people are using instruments to measure the light more precisely than the eye can, and measure it in colors beyond the visible, extending to ultraviolet, infrared, and even x- rays and gamma rays." Viewing the nonvisible light that the sun emits not only gives scientists new ways to examine the sun but new ways to contrast and compare results. With these new tools, scientists have begun to unravel the mystery of sunspots.

What Is a Sunspot? According to George Fischer, a solar astronomer at the University of California, "A sunspot is a dark part of the sun's surface that is cooler than the surrounding area. It turns out it is cooler because of a strong magnetic field there that inhibits the transport of heat via convective motion in the sun. The magnetic field is formed below the sun's surface, and extends out into the sun's corona."

Sunspots are not steady. Most change their size and shape, and all come and go Individual spots may last anywhere from 1 to 100 days. A large group of spots typically lasts 50 days.

Studies of sunspots show an umbra, or dark center, surrounded by a grayish penumbra temperature of the umbra is about 4500 K, compared with the penumbra’s 5500 K.

SOLAR MAGNETISM What causes a sunspot? Why is it cooler than the surrounding photosphere?