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