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Measuring the Diameter of the Sun and the Moon

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Areas of Science
Time Required
Short (2-5 days)
Never, ever look directly at the Sun. You can permanently damage your eyes.
*Note: For this science project you will need to develop your own experimental procedure. Use the information in the summary tab as a starting place. If you would like to discuss your ideas or need help troubleshooting, use the Ask An Expert forum. Our Experts won't do the work for you, but they will make suggestions and offer guidance if you come to them with specific questions.

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You can measure the diameter of the Sun (and Moon) with a pinhole and a ruler! All you need to know is some simple geometry and the average distance between the Earth and Sun (or Moon). An easy way to make a pinhole is to cut a square hole (2-3 cm across) in the center of a piece of cardboard. Carefully tape a piece of aluminum foil flat over the hole. Use a sharp pin or needle to poke a tiny hole in the center of the foil. Use the pinhole to project an image of the Sun onto a wall or piece of paper (Figure 1). Use a ruler to measure the diameter of the projected image. Use your knowledge of geometry to prove that you can calculate the diameter of the Sun using the following proportionality:

Equation for measuring the diameter of the sun using a pinhole and ruler

The equation for measuring the diameter of the sun. The diameter of the sun divided by the distance from the sun to the Earth is equal to the diameter of the image of the sun through a pinhole divided by the distance of the pinhole to the image.

You can also use your pinhole projector to safely view a solar eclipse (by looking at the projected image - do not look directly at the sun, even through the pinhole!). Check out the resources in the Bibliography to learn about more ways to safely view a solar eclipse. If it is a cloudy day, you can also try this experiment with a flashlight or other source of light (Figure 2).

A pinhole projector being used outdoors.
Figure 1. Pinhole projector being used to view a projection of the sun on a piece of paper.

A pinhole projector being used indoors with a flashlight.
Figure 2. Pinhole projector being used indoors with a flashlight.


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MLA Style

Science Buddies Staff. "Measuring the Diameter of the Sun and the Moon." Science Buddies, 26 Mar. 2024, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p026/astronomy/measuring-the-diameter-of-the-sun-and-the-moon. Accessed 14 Apr. 2024.

APA Style

Science Buddies Staff. (2024, March 26). Measuring the Diameter of the Sun and the Moon. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p026/astronomy/measuring-the-diameter-of-the-sun-and-the-moon

Last edit date: 2024-03-26
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