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What is a Light Year?

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What is a Light Year?

The term is familiar, but what exactly is a light year? Contrary to what many think, a light year is actually a measurement of distance, not time. Specifically, a light year is the distance that light travels in one year. Considering light travels at 186,282 miles per second — and nothing travels faster than the speed of light — one light year equals 5.8 trillion miles. Similar units of astronomical measurement include a light minute and light second — the distance light travels in one minute or one second.


1 light year=5,865,696,000,000 miles
1 light minute=11,176,920 miles
1 light second=186,282 miles

To keep numbers manageable, the measurement of a light year is necessary for astronomers in describing the enormous distances between the stars and galaxies of the universe. Proxima Centauri, the closest star to Earth other than the Sun, is 4.2 light years away. The most distant celestial body visible to the naked eye is the Triangulum Galaxy, almost 2.6 million light years away. Stars in the outer reaches of the cosmos are literally billions of light years from Earth.

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Distances From Earth:

Moon: 1.3 light seconds
Sun: 8.3 light minutes
Saturn: 80 light minutes
Sirius: 8.6 light years
Polaris (North Star): 430 light years
Orion Nebula: 1,500 light years
Andromeda Galaxy: 2.2 million light years

While a light year measures distance, there is, however, a time element involved with the concept. For example, Polaris — the North Star — is 430 light years from Earth, which means it takes 430 years for Polaris’ light to reach Earth. So when you look at Polaris, you are seeing it as it looked 430 years ago. You are, in effect, viewing the past. The farther the star is from Earth, the farther back in time you are seeing.

It can be difficult to comprehend the great vastness of the universe. Just our galaxy alone measures 100,000 light years across. Understanding the measurement of a light year can give us an inkling, as well as an appreciation of just how enormous the cosmos is.

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