A light year is one of those space measurements and is similar to how a mile or kilometer measures distance on Earth. Distances in space are so vast, though, that a mile or a kilometer is just too small a number to be useful, because of the huge numbers involved in space travel. Light years work better.
A light year is measured by the time it takes a ray of light to travel a given distance.
While a light year has nothing to do with time as we know it on Earth, it does measure the distance that light travels, or the time it takes the light to move in one year, according to NASA.
Since light moves at about 186,000 miles or about 300,000 kilometers a second, it can travel almost 6 trillion miles or about 10 trillion kilometers in a year.
If people could travel at the speed of light, they would be able to circle the Earth more than seven times in just a second.
In one second, light travels a distance of one light second, and in a year, light travels a distance of one light year.
The moon is a little over one light second from Earth, meaning it would take a beam of light on Earth a little more than a second to reach the moon. The sun, which is 93 million miles from earth, is measured in light minutes and is some eight light minutes away.
Mars is under 25 light minutes from Earth, depending on its orbit around the sun, and the other planets in the solar system are several light hours from Earth.
The Milky Way galaxy, for example, measures about 150,000 light years across. The Andromeda galaxy, the nearest large galaxy, is more than 2 million light years away.
How long does it take to travel a light year? Here's an example. The next closest star after the sun, is called Proxima Centauri. It is just over 4 light years away. If a spacecraft were traveling 38,000 miles per hour, it would still take 80,000 years to reach the star, according to the University of Virginia Physics Department.
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