Measuring the Sun’s Size with a Pinhole Viewer

A simple pinhole viewer is used to project an image of the Sun onto paper, allowing its real diameter to be calculated using geometric ratios and the known distance between Earth and the Sun.

1. Cut a square about 1.5 cm by 1.5 cm from the center of the cardboard.
2. Tape the aluminum foil over the opening so it completely covers the hole.
3. Use a pin or thumbtack to carefully make a tiny hole in the center of the foil.
4. Go outside on a sunny day and hold the cardboard so sunlight passes through the pinhole.
5. Hold a piece of white paper behind the pinhole to catch the Sun’s projected image.
6. Increase the distance between the pinhole and the paper as much as possible while keeping the image clear.
7. Measure the diameter of the circular Sun image on the paper and record it.
8. Measure the distance from the pinhole to the paper using string or a ruler and record it.
9. Use the ratio d/l = D/L to calculate the Sun’s diameter, where d is image size, l is pinhole distance, and L is the distance from Earth to the Sun.

Astronomy at Home Kid's Experiment - Build a Pinhole Camera and Measure the Size of the Sun - Dr Brad Tucker:

Measuring Space - Creating a Pinhole Viewer - UHS STEM Academy:

📄 Size of the Sun - Weber State University: https://physics.weber.edu/schroeder/astro/sizeofsun.html

📄 Measure the Sun’s Size - The Laurence School of Science: https://www.nisenet.org/sites/default/files/catalog/uploads/diy_ss_measure_sun_size.pdf

• Tape the white paper to a book to help keep it upright.
• Use a tablet or smartphone screen as a light source in a dark room to measure the screen size.
• Compare results using different pinhole sizes.
• Have students work in groups and compare calculated values.

• Never look directly at the Sun.
• Do not look through the pinhole at the Sun.
• Keep the viewer pointed away from faces and eyes.
• Supervise younger students when working outdoors.

• Why does the pinhole create a circular image of the Sun? (The Sun is roughly spherical, and the pinhole projects its shape.)
• How does increasing the distance between the pinhole and the paper affect the image size? (The image becomes larger.)
• Why can ratios be used to calculate the Sun’s real size? (The geometry of similar triangles applies.)
• Why is the calculated value different from the actual diameter of the Sun? (Measurement errors and limitations of the setup.)