SUNNY SIDE UP
The most famous solar features are the sunspots. Records from China show that since ancient times, observers have noticed dark spots on the Sun. (These naked-eye observations were made when the Sun was low in the sky or partly obscured by fog, mist, or clouds. Do not try to copy their technique, however— many astronomers in antiquity probably went blind.)
In the early 1600s, Galileo used his telescopes to show that sunspots were not clouds above the photosphere as some claimed, but features on the photosphere itself. Eventually, he and others established that sunspots occur within about
35 degrees of the solar equator.
In the nineteenth century,
Richard Carrington noticed that sunspots near the Sun's equator took about two days less to circle the Sun than did spots in higher latitudes. He concluded that different parts of the Sun rotate at different rates, and that this differential rotation occurs because the
Sun is a gaseous body. It was an important new finding.
Sunspots are places where the photosphere is about
3,000 degrees Fahrenheit (2,000° C) cooler than the surrounding sunscape. This temperature difference makes the spots look darker. Sunspots occur where the Sun's magnetic field has emerged through the photosphere and stopped some of the rising energy from reaching the surface.
Sunspots have a dark central region, called the umbra, surrounded by a striated, lighter-colored halo, known as the penumbra. They vary greatly in size, shape, and complexity, and can appear as single, small, black dots or as complicated groups with so many discrete features that it is difficult to tally them. The largest groups can reach a size of 60,000 miles (100,000 km) across, many times the diameter of the Earth.
In 1769, the Scottish solar astronomer Alexander Wilson noted that the largest sunspots appeared saucer-like when seen near the limb. He proposed that these spots were depressions in the solar surface. Today's astronomers still call this the Wilson Effect, b ut ascribe it to changes in the transparency of the solar gas over a spot when it is seen at an extreme angle.