Sunspots can be larger than Earth.

Tracking spots on the Sun

With a solar viewer, pencil and paper, you can follow sunspots travelling across the face of the Sun.

• 18 September 2006 | Updated 14 October 2011

• What you need
• What to do
• What's happening?

Occasionally the Sun has 'sunspots' on its surface. These are areas that are cooler than the rest of the Sun's surface and appear darker.

The number of sunspots follows a cycle peaking every 11 years. The most recent periods of increased sunspot activity were 1989 and 2000.

Using a solar viewer or images of the Sun on the Internet, you can map the position of sunspots on the surface of the Sun over a period of a week.

In 1612, Galileo noticed that sunspots move across the surface of the Sun. Through his drawing he was able to determine that the Sun rotated once every 27 days.

Sunspot template - click on image for hi-res version.

What you need

To track sunspots you will need:

• a solar viewer or access to the Internet
• a pencil
• copies of the sunspot template.

What to do

1. Photocopy the sunspot template several times to use for drawing sunspots.
2. Each day, at approximately the same time, use your solar viewer or a live image of the Sun on the Internet (for example, the NASA Space Weather website), to see the location and size of sunspots.
3. Draw these spots on a sunspot template using a grey pencil, which you can easily erase if you make a mistake.
4. Over a period of at least one week, you should notice that the position of each sunspot has moved.
5. You can combine these images to create a flicker book, or scan them into a computer and create a slideshow.
6. Choose two of your drawings and compare the position and shape of the sunspots. Has the shape changed? How far have they moved? The vertical lines on the template show the angle from the centre line of the Sun.
7. Measure the angular distance that the sunspots have moved across the Sun and determine how long it would take for them to complete one rotation. To do this divide one rotation (360 degrees) by the distance travelled and multiply it by the time it took to cover that distance. For example, if a sunspot moves 60 degrees in five days, then it would take 30 days to complete one rotation (360/60 multiplied by five).
8. Calculate this value for a number of sunspots and calculate the average. Do you get close to the actual figure of 27 days?

What is happening?

The Sun rotates on its axis once every 27 days. Since the Sun is a ball of gas it does not rotate rigidly like solid planets and moons. In fact, the Sun's equatorial regions rotate faster (taking only about 24 days) than the polar regions (which rotate every 30 days).

Below is an example of a sunspot on the Sun four and a half days (108 hours) apart.

In the first diagram, the sunspot is level with the 20 degree line to the left of the centre line. In the second diagram it is 40 degrees to the right. Therefore the sunspot has moved 60 degrees in 58 hours. It would take 648 hours, or 27 days to complete one rotation.

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