WEBVTT - What is a lightyear?

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<v Speaker 1>Welcome to brain Stuff from how Stuff Works dot com

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<v Speaker 1>it go to meeting dot com slash brain Stuff. Hi.

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<v Speaker 1>I'm Marshall brain with today's question, what is a light year?

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<v Speaker 1>A light year is a way of measuring distance that

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<v Speaker 1>doesn't make much sense because light year contains the word year,

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<v Speaker 1>which is normally a unit of time. Even so, light

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<v Speaker 1>years measure distance. When astronomers use their telescopes to look

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<v Speaker 1>at stars, the distances are gigantic. For example, the closest

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<v Speaker 1>star to Earth besides our Sun is something like twenty

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<v Speaker 1>four trillion miles or thirty eight trillion kilometers away. That's

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<v Speaker 1>the closest star. There are stars that are millions of

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<v Speaker 1>times further away than that. When you start talking about

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<v Speaker 1>those kind of distances, a mile or kilometer just isn't

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<v Speaker 1>a practical unit to use because the numbers get too big.

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<v Speaker 1>No one wants to write or talk about numbers that

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<v Speaker 1>have twenty digits in them. So to measure really long distances.

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<v Speaker 1>People use a unit called a light year. Light travels

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<v Speaker 1>at a hundred eighty six thousand miles per second or

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<v Speaker 1>three hundred thousand kilometers per second. Therefore, a light second

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<v Speaker 1>is a hundred eighty six thousand miles or three hundred

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<v Speaker 1>thousand kilometers. A light year is the distance that light

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<v Speaker 1>can travel in a year, or roughly six trillion miles

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<v Speaker 1>or ten trillion kilometers. That's a long way. So the

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<v Speaker 1>closest star is about four light years away. Using a

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<v Speaker 1>light year as a distance measurement has another advantage. It

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<v Speaker 1>helps you determine age. Let's say that a star is

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<v Speaker 1>a million light years away. The light from that star

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<v Speaker 1>has traveled at the speed of light to reach us. Therefore,

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<v Speaker 1>it's taken the star's light one million years to get here,

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<v Speaker 1>and the light we're seeing was created one million years ago.

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<v Speaker 1>So the star we're seeing is really how the star

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<v Speaker 1>looked a million years ago, not how it looks right now.

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<v Speaker 1>In the same way, our sun is eight or so

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<v Speaker 1>light minutes away. If the sun were to suddenly explode

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<v Speaker 1>right now, we wouldn't know about it for eight minutes,

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<v Speaker 1>because that's how long it would take for the light

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<v Speaker 1>of the explosion to get here. Do you have any

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<v Speaker 1>ideas or suggestions for this podcast. If so, please send

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<v Speaker 1>me an email at podcast at how stuff works dot com.

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