Let's Measure the Diameter of the Sun!

You can measure the diameter of the sun from your own backyard! You'll just need a few simple tools and a little clever math. A really long tape measure is NOT required!

As always, NEVER look directly at the sun!!!

[ Show Transcript ]

Announcer: Frostbite Theater presents... Cold Cuts! No baloney!

Joanna and Steve: Just science!

Joanna: Hi! I'm Joanna!

Steve: And I'm Steve!

Joanna: Today, we're going to measure the diameter of the son!!

Steve: Awwww...

But, not your son!

This sun!

Joanna: As you can see, we went a little overboard on the set-up. All you really need is a piece of aluminum foil and a piece of paper.

Poke a small hole in the foil and use it to project the sun's image onto the paper. Carefully measure the diameter of the sun's image and the distance from the foil to the paper.

Steve: The ratio of the image's diameter to its distance to the pinhole is equal to the ratio of the sun's diameter to its distance to the pinhole. Our image is about a centimeter across and it's 100 centimeters away from the pinhole. If we knew the distance to the sun, we could figure out the sun's diameter.

Joanna: Well, we do know the distance to the sun. On average, it's about 150 million kilometers away.

How do we know this? Early methods for measuring it included timing the transit of Venus, or measuring the parallax of the asteroid Eros, and then applying Kepler's Laws. Today, we bounce radar off the inner planets and then apply Kepler's Laws.

Steve: Now that we know the distance to the sun, we find that its diameter is about 1.5 million kilometers, which is close to the accepted value of about 1.4 million kilometers.

Joanna: Thanks for watching! I hope you'll join us again soon for another experiment!

Steve: Welcome back!

Joanna: Thank you!

Steve: It's nice that you're finally fitting in-frame again!

Joanna: Hey, now! Shut your mouth!

Steve: Sorry, but you were BIG!

As an added challenge, can you prove that the formula used to determine the diameter of the sun is, in fact, correct?

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This page is maintained by Steve Gagnon.