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Astronomical Units & Light Years: Definition & Examples

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  • 0:01 Measuring Distance
  • 1:19 What Is a Light Year?
  • 1:56 What Is an Astronomical Unit?
  • 3:31 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain why we need special astronomical ways of measuring distance. You will also be able to state the meaning of the terms astronomical unit, light year and parsec. A short quiz will follow.

Measuring Distance

Measuring the distance across your living room is pretty easy. Just grab a tape measure and off you go. Measuring the distances between stars and galaxies... not so much. We can't physically grab a tape measure and pull it across huge distances to see how far away objects in the sky are from each other. Instead, we have to use some clever math. Sometimes we can use geometry by measuring how the position of stars vary during the year. For further away objects, we have to get cleverer, and analyze the light with computers.

But, whatever the distance turns out to be, one thing's for sure - it will be HUGE. Or 'astronomical' if you like. The distance to the sun is 93 million miles. The distance to the nearest star, Alpha Centauri, is 25,670 billion miles. The distance to the center of the galaxy is 153,400 trillion miles. And it just gets more ridiculous from there. These distances are so incredibly huge that miles just doesn't cut it. It's like using millimeters to measure the length of an entire country. It just doesn't make sense.

So instead, astrophysicists have come up with some alternative ways to measure huge distances. Instead of miles, let's talk about light years and astronomical units.

What Is a Light Year?

A light year might sound like it's a period of time, but it's actually not. It's a way of measuring distance. A light year is the distance light travels in one year. Light moves super-fast, so that's a really big distance. One light year is almost six trillion miles.

In light years, the numbers get a lot better. The sun is eight light MINUTES away - it takes eight minutes for light to get to us from the sun. The nearest star is four light years away. And the center of the galaxy is 26,000 light years away. Much more reasonable!

What Is an Astronomical Unit?

But for some things, miles and kilometers are too small, but light years are also too big. If you're a planetary astronomer, focusing on the objects in our solar system, light years might not float your boat. Instead, you might want to use astronomical units, or AUs. An astronomical unit is the average distance between the earth and the sun. So the distance to the sun is by definition one AU.

Astronomical units also lead to another distance measurement: parsecs. A parsec is the distance at which one AU subtends an angle of one second of arc. That's probably a bit confusing. But maybe this diagram will help:

Diagram explaining a parsec
parsec diagram

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