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How Far Is A Light-Year?

October 31, 2010

A team of planet hunters spotted a “nearby” earth-like planet called Gliese 581. The planet is in the “Goldilocks zone” of space around its red dwarf star where surface temperatures are neither too hot nor too cold, but just right for liquid water, and therefore life, to form.

Unless we develop Warp-Drive or discover a wormhole between Earth and Gliese 581, we won’t be able to travel there. The problem is that it is 20 light-years away. Not everyone understands what a light-year is so I thought I’d explain it. It is not a measure of time, but a measure of extremely long distance.

The speed of light is 186,000 miles per second. A light-year is the number of miles that light would travel in a year. To calculate this, you would multiply 186,000 x 60 seconds per minute x 60 minutes per hour x 24 hours per day x 365 days per year. The answer is 5,865,696,000,000 miles, or approximately 5.9 trillion miles. Gliese 581 is 20 light-years away so multiply again by 20 and you find that it is 117.3 trillion miles from earth.

To put this in perspective, it took the Apollo astronauts about four days (96 hours) to reach the moon which is 240,000 miles away. Dividing 240,000 miles by 96 hours, we find that their average speed was 2500 miles per hour. At that speed it would take us a mere 5.4 million years to get to Gliese. If we could travel at the speed of light it would still take 20 years. Now you know.


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