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Measuring the Diameter of the Sun and the Moon


Areas of Science
Time Required
Short (2-5 days)
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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. 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.

Important Safety Note: Never, ever look directly at the Sun. You can permanently damage your eyes (UC Regents, 2001).


UC Regents, 2001. "Finding the Size of the Sun and Moon," The Center for Science Education @ Space Sciences Laboratory, University of California, Berkeley [accessed July 31, 2006] http://cse.ssl.berkeley.edu/AtHomeAstronomy/activity_03.html.
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MLA Style

Science Buddies Staff. "Measuring the Diameter of the Sun and the Moon." Science Buddies, 12 Jan. 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p026/astronomy/measuring-the-diameter-of-the-sun-and-the-moon. Accessed 20 May 2022.

APA Style

Science Buddies Staff. (2020, January 12). 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: 2020-01-12
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