In an effort to understand and explain nature without prejudice, scientists rely on the processes of scientific methods. Careful and systematic observations of the natural world can provide valuable data from which to interpret natural phenomena. In the early 17^th century, however, most beliefs about the natural world were based on philosophy and religion. Galileo was part of a scientific revolution that changed this, as people began taking a more robust approach to observation, experimentation, the use of mathematics, and the understanding of nature.

In 1611, several people used the newly invented telescope to observe dark areas on the face of the Sun. At the time, no one had any idea what the dark spots might be. A German mathematician, Christoph Scheiner, hypothesized that the spots were satellites orbiting the Sun, a theory that preserved the accepted belief that the Sun, along with all heavenly bodies, was perfect and without blemishes. In contrast, Galileo's scientific observations of the dark spots led him to believe that they were actually located on the surface of the Sun.

As he studied the mysterious spots, Galileo took careful notes and made daily illustrations of the Sun's appearance. Galileo noticed that when the spots approached the edge of the Sun's disk, they appeared to become narrower and their movement slowed, both of which are visual illusions of a rotating sphere. Because Galileo's eyes could not perceive the three-dimensional movement of the spots, as the spots rotated toward or away from him they appeared to move more slowly when they were at the edge of the disk. In addition, foreshortening — a visual effect that distorts shape based on perspective — appeared to flatten the spots near the edge of the Sun. Moreover, the spots had very irregular shapes and would sometimes appear and disappear within the disk, something that orbiting satellites would not do. Thus, Galileo realized that the sunspots must be on the surface of the rotating Sun.

Today we know that Galileo's scientific approach directed him to the correct understanding that sunspots are surface features of the Sun. Sunspots are relatively cool areas — about 1000 K (1800°F) cooler than the typical 5800 K (9980°F) temperature of the Sun's surface — and so appear as dark spots. They are not permanent features on the surface, but are effects of the Sun's dynamic magnetic field, and they regularly change in location, number, and shape.

Construct and evaluate graphs of the current sunspot cycle in this NOVA classroom activity.

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