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What is a light-year?

It's more than just the way we measure the cosmos.
Image: Light year
Even though a light-year might sound like a way we measure time, it's actually a way to measure the distance of far-off galaxies.Pitris / Getty Images/iStockphoto

The light-year is an indispensable unit of measurement for anyone trying to understand outer space. Get a group of astronomers talking, and you’ll hear them say “light-years” about as often as you hear football fans mention “yards.”

For nonscientists, though, the light-year can be a puzzling concept. It’s a unit of distance even though it might sound like a unit of time. Adding to the confusion, sci-fi movies and TV shows routinely bungle the concept.

Even the original "Star Trek" series got light-years wrong. In a 1968 episode, the alien princess Elaan of Troyius tells Captain Kirk she’d rather hide in her room for “10 light-years” than come out and speak to him. Oops.

How far is a light-year?

A light-year is the distance a beam of light travels in a vacuum in one year. It’s important to specify a vacuum because light slows as it passes through any kind of matter. (In water, for instance, it travels about 25 percent slower.) Most of the universe is a near-perfect vacuum, so astronomers can generally assume that light is moving at its top speed.

Light travels at 299,792,458 meters (186,282.397 miles) per second. Multiply that number by the number of seconds in a year (31,557,600), and you get your answer: One light-year is 9,460,730,473,000 kilometers, or 5,878,625,373,000 miles.

That’s probably more digits than you were looking for. Round it off to 6 trillion miles, and you’re still close enough for any in-the-know nerd conversation.

Why do astronomers measure distance in light-years?

Familiar units like kilometers and miles are absurdly small for describing the vastness of the cosmos. Take the example of Proxima Centauri, the nearest star beyond the sun. You could say that it's about 24,900,000,000,000 miles away — or save a lot of breath and call the distance a tidy 4.24 light-years.

Most of the stars you see at night lie within a few hundred light-years of Earth. The Milky Way galaxy in which we live happens to be a nice, round 100,000 light-years across. The Andromeda Galaxy is 2.54 million light-years away — a big number but still a whole lot more manageable than “14 quintillion, 900 quadrillion miles.”

So light-years are only a measure of distance, not time?

Strictly speaking, yes. A light-year is a unit of distance, just like feet and inches. If you want to avoid any confusion, you can stop right here.

But there’s another fascinating side to the light-year story. Since light moves at a finite speed, everything you see is outdated: Your view of the world is actually an image of what things looked like at the instant their light began traveling toward you.

If you're staring across a room, the lag is just billionths of a second — utterly imperceptible. Look at the moon, and you see it as it was about a second-and-a-half ago. Watch a sunset, and you're seeing the sun of 8.3 minutes ago.

The effect is far more pronounced for the stars. They're light-years away, so when you look at them you're looking years into the past.

Sirius, the brightest star in the sky, lies 8.6 light-years from Earth, which means the light you’re seeing left it 8.6 years ago. Put another way, we always see Sirius as it was 8.6 years ago. Deneb, a prominent summer star in the constellation Cygnus, is about 2,500 light-years away. Its twinkling glow was already en route to us while Aristotle was alive.

And remember, other galaxies are even more distant. Powerful telescopes like Hubble can detect galaxies whose light has been traveling our way for billions of years. Thus those ethereal Hubble images show us galaxies as they were billions of years ago — in some cases, from a time before Earth came to be.

In short, a light-year describes distance only — but light itself provides a sort of time machine, allowing us to observe the cosmos as it was long ago.

How do we know the speed of light?

The crucial insight came from Ole Rømer, a 17th-century Danish astronomer. During the 1660s, he was studying one of Jupiter’s moons, Io, when he noticed something odd: When Jupiter and Earth were at their greatest distance from one another, Io would slip into Jupiter’s shadow a few minutes later than astronomers predicted. When the two planets were closest together, the event seemed to occur a few minutes early.

Rømer realized that the delay had nothing to do with Io. Rather, it was an illusion caused by the time it takes for light to span the extra distance when Earth and Jupiter are on opposite sides of the sun. His calculations showed that light travels 131,000 miles a second — a remarkably good estimate, considering that he was doing this work just 60 years after the invention of the telescope.

Who came up with the term light-year?

“The concept of light speed as a measurement of distance already happened by the end of the 17th century, following from the discovery of the finiteness of light speed by Rømer,” says Frédéric Arenou, an astronomer and science historian at the Paris Observatory.

People latched on to the idea so quickly that it’s impossible to credit any individual with certainty. Arenou points to one good candidate, however: the English scholar Francis Roberts, who in 1694 mused that “Light takes up more time in travelling from the stars to us, than we in making a West-India Voyage.”

At first, these ideas were necessarily vague because scientists had only a rough idea of how far away the stars are. The breakthrough moment came in 1838, when German astronomer Friedrich Bessel measured the exact distance to the star 61 Cygni. In describing the huge number he got, Bessel wrote that “light employs 10.3 years to traverse this distance.” That’s the closest thing to a specific moment when the specific concept of the light-year was born.

Within a couple of decades, the light-year was commonplace in popular science writing. But Arenou says professional astronomers long resisted using the term — and for a surprising reason. They considered the light-year insufficiently scientific, since it cannot be measured directly.

How do astronomers measure distances in light-years?

Bessel reckoned the distance to 61 Cygni by observing its parallax, the apparent back-and-forth motion in the sky caused by Earth’s motion around the sun.

Parallax is still one of astronomers’ most powerful tools for measuring distance. The European Space Agency spent 650 million Euros ($750 million) on the Gaia space telescope, which is currently using parallax to measure the distances to more than a billion stars across our galaxy.

Beyond the Milky Way, where parallaxes are too small even for Gaia to detect, astronomers calculate distances by observing certain types of variable stars or brilliant supernova explosions. Those approaches still rely on parallax measurements as their point of reference, however.

Because their work is so deeply rooted in parallax, astronomers commonly use a second distance unit, the parsec (“parallax arc-second,” a small angle). One parsec equals 3.26 light-years. Sometimes the parsec is more scientifically relevant, but researchers often switch between the two terms for no obvious reason except style.

Unfortunately, the “sec” in parsec sounds like a unit of time, leading to more confusion. In the 1977 movie "Star Wars: A New Hope," Han Solo brags that he “made the Kessel Run in less than 12 parsecs.” Star Wars fans have tied themselves in knots trying to explain away that obvious goof. Better to stick with the light-year. It’s all the astronomy you need — and lets you dabble in time travel, too.