Stanford Solar Center
About the SunFor StudentsFor EducatorsSpace Weather Monitors
Ancient ObservatoriesSolar FolkloreSolar Art & Literature

portrait of Galileo Are Those Sunspots Really on the Sun?


  1. What can you say about the shapes of the sunspots? Do they remain constant?
  2. Look at the shape of one sunspot as it appears on the edge (limb) of the Sun's image. What happens to its width-to-height ratio as it moves across the disk, and when it again approaches the limb on the other side? Why do you think that is?
  3. Do sunspots always appear and disappear on the solar limb?
  4. Look at your graph. Does the "movement" of sunspots across the disk (in centimeters per day) remain constant with longitude?
  5. If the distances traveled, and hence the speeds, were different, in what areas of the Sun did they appear faster? In what areas did they appear slower?
  6. Why do you think the spots appear to move at different speeds the way they do?
  7. In your groups, discuss whether or not you think sunspots are features on the Sun's surface. Suppose that the spots are objects in orbit some significant distance away from the Sun. Would their speeds appear to change much as they went past the limb and then across the center of the solar disk?

    If you have difficulty visualizing this, try a simple experiment. Draw a dot on a basketball, and rotate the ball around its axis, such that the dot appears to travel horizontally when you look at it from the side. While spinning the ball at a constant rate, observe the apparent changes in speed of the dot. Now, put the ball down on a table, and sit across the room. Have another person hold a tennis ball a few feet from the basketball and slowly move it in an orbiting motion around the basketball. Observe whether the speed of the tennis ball varies much while it passes in front of the basketball.

    Then, check out this SOHO image of Mercury's transit across the solar disk!

Extra questions:

  1. Does your sunspot speed graph in centimeters per day show angular or linear velocity?
  2. Does the angular velocity of the sunspots remain fairly constant? Why or why not?

Extra assignment:

If you were Galileo, how would you mathematically prove the spots are actually on the Sun?

©2008 by Stanford SOLAR Center · Permitted Uses · Credits