What Is a Light-year?

So what exactly are light-years — or rather, how far is one light-year? Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light-year is the distance that light travels in a year, or:

186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year


One light-year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). Light-years are a long way!

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Using a light-yearas a distance measurement has another advantage — it helps you determine age. Let’s say that a star is 1 million light-years away. The light from that star has traveled at the speed of light to reach our planet. Therefore, it has taken the star’s light 1 million years to get to our planet, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks at this moment from our viewpoint. In the same way, our sun is 8 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 is how long it would take for the light of the explosion to get to Earth.

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Originally Published: Apr 1, 2000

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