Measuring the Diameter of the Sun and the Moon

Summary

Areas of Science

Astronomy

Difficulty

Method

Scientific Method

Time Required

Short (2-5 days)

Safety

Never, ever look directly at the Sun. You can permanently damage your eyes.

Credits

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https://www.youtube.com/watch?v=JrV_f_omYHY

Abstract

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:

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).

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

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

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Careers

If you like this project, you might enjoy exploring these related careers:

Astronomer

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Astronomers think big! They want to understand the entire universe—the nature of the Sun, Moon, planets, stars, galaxies, and everything in between. An astronomer's work can be pure science—gathering and analyzing data from instruments and creating theories about the nature of cosmic objects—or the work can be applied to practical problems in space flight and navigation, or satellite communications. Read more

Mathematician

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Career Profile

Mathematicians are part of an ancient tradition of searching for patterns, conjecturing, and figuring out truths based on rigorous deduction. Some mathematicians focus on purely theoretical problems, with no obvious or immediate applications, except to advance our understanding of mathematics, while others focus on applied mathematics, where they try to solve problems in economics, business, science, physics, or engineering. Read more

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