A sunspot

Sunspots are regions on the solar surface that appear dark because they are cooler than the surrounding photosphere, typically by about 1500 K (thus, they are still at a temperature of about 4500 K, but this is cool compared to the rest of the photosphere). They are only dark in a relative sense; a sunspot removed from the bright background of the Sun would glow quite brightly.

Basic Features of Sunspots

The largest sunspots observed have had diameters of about 50,000 km, which makes them large enough to be seen with the naked eye. Sunspots often come in groups with as many as 100 in a group, though sunspot groups with more than about 10 are relatively rare. There are well established methods for measuring the number of sunspots. Here, for example, is a table giving the daily count of observed sunspots in the year 1996.

Sunspots develop and persist for periods ranging from hours to months, and are carried around the surface of the Sun by its rotation (a fact known to Galileo). The images on this page show a single sunspot and a complex sunspot group. A typical sunspot consists of a dark central region called the umbra and somewhat lighter surrounding region called the penumbra

A complex sunspot group.

The Solar Rotational Period

Historically, the first measurements of the period for solar rotation were made by tracking sunspots as they appeared to move around the Sun. Galileo used this method to deduce that the Sun had a rotational period of about a month.

Because the Sun is not a solid body, it does not have a well defined rotational period. Modern measurements indicate that the rotational period of the Sun is about 25 days near its equator, 28 days at 40 degrees latitude, and 36 days near the poles. The rotation is direct, that is, in the same sense of the motion of the planets around the Sun.

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