How many sunspots are there on the sun?

### And how big are they?

This answer is courtesy of Dana Longcope, Montana State University.

Roughly how many sunspots are on the Sun? There were 17 on a map made last Saturday, October 18, 1997. On other days, as many as 200 sunspots are visible, or else as few as zero. (More sunspots as the year 2000 approaches.)

How big do they get? About 10,000 miles across for a medium-large sunspot.

Often in science, simple questions can have complicated answers. This question is simple, but the answers are not simple for several reasons.

The number of sunspots on the Sun is always changing. One reason for this is that a sunspot only lasts a week or two. A sunspot is "born" when a small piece of the Sun's surface suddenly grows dark, seemingly for no reason. (What has really happened is that underneath the surface a very strong magnetic field "turned on". We do not fully understand why this happens, but we have many good ideas.) After several weeks, the sunspot "breaks" into pieces and fades away. This is why we say a sunspot lasts only several weeks.

As the sunspot is "breaking up", the Sun is rotating about its axis, taking about 27 days to spin completely around. So even before a sunspot fades away, it might seem to disappear from view as it rotates around to the far side of the Sun. Other sunspots will come into view as they rotate onto the near side from the far side. If sunspots never changed, then in one month we would see all the sunspots there were, and then each month after that we would just see the same ones over and over again. But they do change, so each month we see different sunspot numbers each month.

So the number of sunspots on the Sun at any time depends on how many have been born lately and have not faded away yet. Looking at a map of the Sun made on Saturday, October 18, I see 17 different sunspots clustered into three close groups. One group is just about to disappear onto the far side. It is likely that these sunspots will fade away before they make it back onto our side again, about the date of November 3. From what I have said above, you probably guessed that making maps of the Sun is sort of like making maps of clouds: the maps change and you will need to make a new map very soon. And a new map of the Sun is made every day, for the special purpose of counting the number of sunspots. (These maps are made at special telescopes because it is dangerous to look at the Sun with your eyes.)

For over 150 years, observers have recorded the number of sunspots they found on the Sun on each day. Some days there were no sunspots, and some days there were as many as 200 sunspots! The Sun has times when sunspots are born often, and the number found might be very high. These times are called Solar Maximum and that happens every 11 years or so. The last Solar Maximum was in 1989, so the next one should be in the year 2000. At Solar Maximum, there will be up to 200 sunspots on the Sun at one time.

How big is a sunspot? Sunspots look like tiny specks on the Sun, but that is because the Sun itself is so BIG. (Really, really big). In a medium-large sunspot (they come in many sizes) the dark middle region (called the umbra) measures about 10,000 miles across. This is wider than the Earth, and three times bigger than the distance from New York to Los Angeles. A really big sunspot can be up to 30,000 miles across, and a whole group of sunspots can be 100,000 miles long.

You may be interested in seeing a Web site where daily maps of sunspots are made. This site shows the daily sunspot drawing observations at Mt. Wilson Observatory.

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More about sunpots, from Amara Graps.

Some people hope to predict sunspots, but for now, predicting sunspots is a little like trying to predict the weather.

If are interested in daily sunspot numbers for any time in history back to the year 1818, you can find them at the Sunspot Index Data Center (SIDC) in Belgium, which is the world data center for sunspot index of daily sunspot numbers.

If you would like to learn more about how big are the features of the Sun, you may jump to the Solar Center's Web page of comparison activities. We also have a selection of Web pages for measuring the Sun's rotation using sunspots ( and race your friends! ), and for learning how Galileo knew that the sunspots were on the Sun.

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Special Thanks to D. Longcope