What Is a Light-year?

By: Valerie Stimac  | 
light-year
The word "year," is normally a unit of time, but light-years measure distance. M. Aurelius/Shutterstock

Light-years are a way of measuring distance in space. 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.

Here on Earth we are used to measuring all distances in either inches/feet/miles or centimeters/meters/kilometers, depending on where you live and what method of measurement is used. Thanks to school, most people know how long a foot or a meter is — we are comfortable with these units because we use them every day. Light-years, not so much. Same thing with miles and kilometers — these are nice, human increments of distance that our minds can easily understand. Light-years, not so much.

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Distances Light Travels in Space Are Gigantic

When astronomers use their telescopes to look at stars, it's different. The distances in the universe are gigantic — or even within our Milky Way galaxy. For example, the closest star to Earth (besides our sun) is called Proxima Centauri, and it is something like 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. That's the closest star, and there are so many zeros that most people aren't sure which -illion suffix to use (it's 24 trillion miles, or 38 trillion kliometers).

There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn't a practical unit to use because the numbers get too big, even when you step up to billions and trillions of them. No one wants to write or talk about numbers that have 20 digits in them! That's where light-years come in.

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Who Coined the Term 'Light-year'?

To measure really long distances and how fast light travels, astronomers and other scientists use a unit called a light-year.

The term "light-year" was first coined by German scientist, astronomer, mathematician and physicist Friedrich Bessel in 1838, when he measured the distance from Earth to a star called 61 Cygni, and came up with a distance of 660,000 times Earth's orbital radius.

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He calculated that light would take about 10 years to get to 61 Cygni from Earth. Later, astronomers adopted the term and "light-years" quickly became standard to help make sense of the massive distances between basically everything in our vast universe.

How Far Is One 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

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

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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