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Speaker 1: Welcome to brain Stuff from how Stuff works dot com
Speaker 1: where smart happens. Hi. I'm Marshall Brain with today's question,
Speaker 1: what is a light year? A light year is a
Speaker 1: way of measuring distance that doesn't make much sense because
Speaker 1: light year contains the word year, which is normally a
Speaker 1: unit of time. Even so, light years measure distance. When
Speaker 1: astronomers use their telescopes to look at stars, the distances
Speaker 1: are gigantic. For example, the closest start of Earth besides
Speaker 1: our sun is something like twenty four trillion miles or
Speaker 1: thirty eight trillion kilometers away. That's the closest star. There
Speaker 1: are stars that are millions of times further away than that.
Speaker 1: When you start talking about those kind of distances, a
Speaker 1: mile or kilometer just isn't a practical unit to use
Speaker 1: because the numbers get too big. No one wants to
Speaker 1: write or talk about numbers that have twenty digits in them.
Speaker 1: So to measure really long distances, people use a unit
Speaker 1: called a light year. Light travels at a hundred eighty
Speaker 1: six thousand miles per second or three hundred thousand kilometers
Speaker 1: per second. Therefore a light second is a hundred eighty
Speaker 1: six thousand miles or three hundred thousand kilometers. A light
Speaker 1: year is the distance that light can travel in a year,
Speaker 1: or roughly six trillion miles or ten trillion kilometers. That's
Speaker 1: a long way, so the closest star is about four
Speaker 1: light years away. Using a light year as a distance
Speaker 1: measurement has another advantage. It helps you determine age. Let's
Speaker 1: say that a star is a million light years away.
Speaker 1: The light from that star has traveled at the speed
Speaker 1: of light to reach us. Therefore, it's taken the star's
Speaker 1: light one million years to get here, and the light
Speaker 1: we're seeing was created one million years ago. So the
Speaker 1: star we're seeing is really how the star looked a
Speaker 1: million years ago, not how it looks right now. In
Speaker 1: the same way, our sun is eight or so light
Speaker 1: minutes away. If the sun were to suddenly explode right now,
Speaker 1: we wouldn't know about it for eight minutes, because that's
Speaker 1: how long it would take for the light of the
Speaker 1: explosion to get here. For more on this and thousands
Speaker 1: of other topics, does that how stuff works dot Com
Speaker 1: and don't forget to check out the brain stuff blog
Speaker 1: on the house stuff works dot com home page. You
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Speaker 1: has arrived down at it today on iTunes. This episode
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