milky way

Photo via John A Davis / Shutterstock (Licensed) Remix by Max Fleishman

And why do astronomers use parsecs?

Light-years are a common metric in astronomy, but what are they and how long are they? A better way to phrase that question would actually be, how far are they? Light-years are measures of distance, not time, and they correspond to the distance traveled by a single particle of light over the course of the year.

Light travels super fast, at 186,282 miles per second, or about 670 million miles per hour. There are about 8,766 hours per year, so light travels 5.9 trillion miles per year. That means a light-year is equal to 5.9 trillion miles.

But how far is this really? How long would it take for you to say, do a road trip equivalent to one light-year?

The longest highway in America is U.S. Route 20, which is 3,365 miles in length. Let’s say you drive about eight hours per day at an average speed of 65 miles per hour. At that rate, you’ll make it from Massachusetts to Oregon in about six and a half days.

Going at the same rate, it’s going to take you roughly 11.3 billion days to make a journey equivalent to one light-year. The average American human life is currently estimated at 78.74 years, which translates into 28,740 days. So you would have to live about 400,000 times longer than the average American to get there.

If you were arriving there now, you would have started your journey somewhere around the time that mammals were really coming into their own on Earth, but way, way before modern humans were around.

Another way to think of it: You’re going to need long-term help to do this road trip so let’s say you start having kids. The average age for an American woman to have her first child these days is 26. So if you’re having kids every 26 years, your great-great-great-great….great grandchild will finally get there about 1.2 million generations later.

Why do we use light-years to measure distance in space?

If you’re having trouble wrapping your mind around how staggeringly far a light-year is, you’re not alone. Humans, in general, struggle to conceptualize very large numbers. For example, try to imagine how big of a room you’d need to comfortably seat a 10-person movie theater. Now do the same thing with 100 people. Now 1,000, and so on. After a certain amount, it becomes harder and harder to really visualize 100,000 people in a room versus 1,000,000.

But the problem with thinking about space, and distances between things in space, is that space is really, really big. The light-year was first used as a unit of measurement by astronomer Friedrich Bessel in 1838, though he didn’t call it a light-year at the time. He was trying to use the distance light travels in a year to demonstrate the distance between the Earth and another star, 61 Cygni.

Let’s go back to the one-light-year road trip. Once you got that far, you would reach pretty much nothing. You’d pass by Pluto about 8 million days into your trip. But that part of the journey would only be about 0.07 percent of the whole trip. Our nearest celestial neighbor, Proxima Centauri, is four light-years away.

Most astronomers these days don’t even use light-years as the unit of distance for space; they use our section's namesake, parsecs. We couldn’t find any consistent reason why. Some say that parsecs, which are equivalent to about 3.26 light-years, are more accurate. Others say it’s simply the convention.

The real takeaway here is that if you’re doing any sort of interstellar travel, you’re going to need a faster ship, such as the Starship Enterprise, or the Heart of Gold.

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