WEBVTT - What  is a light year?

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

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<v Speaker 1>where smart happens. Hi. I'm Marshall Brain with today's question,

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<v Speaker 1>what is a light year? A light year is a

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<v Speaker 1>way of measuring distance that doesn't make much sense because

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<v Speaker 1>light year contains the word year, which is normally a

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<v Speaker 1>unit of time. Even so, light years measure distance. When

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<v Speaker 1>astronomers use their telescopes to look at stars, the distances

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<v Speaker 1>are gigantic. For example, the closest start of Earth besides

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<v Speaker 1>our sun is something like twenty four trillion miles or

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<v Speaker 1>thirty eight trillion kilometers away. That's the closest star. There

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<v Speaker 1>are stars that are millions of times further away than that.

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<v Speaker 1>When you start talking about those kind of distances, a

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<v Speaker 1>mile or kilometer just isn't a practical unit to use

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<v Speaker 1>because the numbers get too big. No one wants to

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<v Speaker 1>write or talk about numbers that have twenty digits in them.

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<v Speaker 1>So to measure really long distances, people use a unit

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<v Speaker 1>called a light year. Light travels at a hundred eighty

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<v Speaker 1>six thousand miles per second or three hundred thousand kilometers

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<v Speaker 1>per second. Therefore a light second is a hundred eighty

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<v Speaker 1>six thousand miles or three hundred thousand kilometers. A light

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<v Speaker 1>year is the distance that light can travel in a year,

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<v Speaker 1>or roughly six trillion miles or ten trillion kilometers. That's

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<v Speaker 1>a long way, so the closest star is about four

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<v Speaker 1>light years away. Using a light year as a distance

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<v Speaker 1>measurement has another advantage. It helps you determine age. Let's

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<v Speaker 1>say that a star is a million light years away.

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<v Speaker 1>The light from that star has traveled at the speed

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<v Speaker 1>of light to reach us. Therefore, it's taken the star's

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<v Speaker 1>light one million years to get here, and the light

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

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

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

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

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<v Speaker 1>minutes away. If the sun were to suddenly explode right now,

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<v Speaker 1>we wouldn't know about it for eight minutes, because that's

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<v Speaker 1>how long it would take for the light of the

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<v Speaker 1>explosion to get here. For more on this and thousands

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<v Speaker 1>of other topics, does that how stuff works dot Com

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<v Speaker 1>and don't forget to check out the brain stuff blog

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