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