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Laboratory for Atmospheric and Space Physics

Classroom Experiment: Sunspot Activity

The solar activity is observable with the appearance and disappearance of active magnetic regions on the Sun. In the visible spectral range, these active regions appear as dark regions and are called sunspots. The sunspots are associated with large magnetic fields erupting from the Sun and often appear in pairs of opposite magnetic polarities. The bright regions surrounding a sunspot are called faculae. This simple experiment examines sunspot activity using a pinhole camera.

Materials needed

Mailing tube (~1 m long), cardboard box, aluminum foil, scissors, white tracing paper (thin), tape, pencil.


  1. With thick aluminum foil or with 2 layers of foil, make a tiny hole (about 0.5 mm) with a sharp pencil. Remove any end caps on the mailing tube. Attach this foil to one end of the mailing tube with the hole in the center of the tube. It is best to wrap this foil over the end of the mailing tube and secure tightly with tape so that light can only enter into the tube through the tiny hole.
  2. Cut a hole in the middle of the cardboard box bottom so that the mailing tube can slide through this hole. Trace the mailing tube diameter onto the bottom of the box before cutting so that the mailing tube is a tight fit into the box. This box serves as a shield to keep stray light off the back end of the mailing tube.
  3. Place a sheet of white tracing (thin) paper over the other end of the mailing tube and tape down. Your mailing tube is now a pinhole camera!
  4. On a clear, sunny day, point the mailing tube’s end with the tiny hole towards the Sun. Then pivot the back of the mailing tube until the image of the Sun falls onto the paper. Initial alignment is easier without the paper fully attached so one can peer into the tube to see where the solar image is going.

    WARNING: Do not look directly at the Sun as its intense brightness can damage your eyes.
  5. With an assistant or prop to steady the mailing tube, trace the image of the Sun (circle)and any dark spots (the sunspots) on the paper. If the solar image is difficult to see, one can use the pinhole camera inside from a sun-facing window that allows the mailing tube to peer past its curtains or shades. With a 1 m mailing tube, the solar diameter will be about 10 mm on the paper. Label the current date and time on the paper. You will need to trace lightly so as not to tear the paper.
  6. Count the number of sunspots and note the location of each sunspot.
  7. On the next day or a few days later, repeat steps 3-6 with a fresh copy of paper. Repeat as often as you want on other days. It is best to repeat the measurement at about the same time of day.


  1. How many sunspots are there on each day?
  2. Did the number of sunspots change on different days?
  3. Did the sunspots move from one day to another?
  4. How dark are the sunspots compared to the quiet Sun background?
  5. Can you see the bright faculae region surrounding the sunspots?
  6. Is the limb (outer edge) of the Sun darker or brighter than the center?
  7. How much is the total area of the sunspots compared to the area of the Sun?

Discuss with students

  1. Discuss how sunspots evolve.
  2. Discuss how their sunspot measurements appear similar to and different from solar images taken by ground-based and space-based telescopes.
    You can examine the daily solar images obtained at different wavelengths from:

  3. Discuss how to calculate the rotation speed of the Sun by measuring the movement of the sunspots. This calculation is simpler and more accurate if tracking a sunspot at the center of the solar disk with two measurements taken 1 day apart. For this approach, use:
    • speed = distance / time where:

      • time (1 day) = 86400 seconds
      • distance on Sun = 1.4 x 106 km x (distance moved on paper ÷ diameter of disk on paper)