Measurement

# A Parsec Is a Real Astronomical Unit of Measurement

Now, hang on just one parsec ... or maybe not. Though a parsec may sound like a derivative of "second," it actually isn't. And that's why the famed line about the Millennium Falcon in Star Wars — "It's the ship that made the Kessel Run in less than 12 parsecs" — doesn't make sense. The true meaning of the word is way cooler anyway.

## Larger Than Life

For starters, a parsec is a unit of measurement, not a unit of time. Sorry, Han. It's a very, very large unit of measurement at that. Specifically, one parsec is equivalent to 3.26 light years, or 19 trillion miles (31 trillion kilometers). If this seems excessively large, that's because it sure is. But when it comes to measuring astronomically large distances between objects beyond our solar system, excessively large is just right.

That's right, objects within our own, expansive solar system barely sit far enough apart to require parsecs to measure their distances. For example, the distance between the Earth and the sun is exactly one astronomical unit, called an AU. One parsec is 206,265 AU. If you went to the sun and back 206,265 times, you would have successfully traveled a distance totaling one parsec. Impressive.

## Parse It Out

Why name something a parsec when it has nothing to do with seconds or par on a golf course? Its origins make sense, even if they're not the most obvious. The word itself comes from two words: parallax and arcsecond. Parallax describes when an object's location seems to have changed because your location changed. Astronomers use parallax to judge the distance of objects in the sky; if you know how far the Earth has moved and can measure how far the object moved across its starry backdrop, you can judge its distance. Astronomers aren't the only ones, however — you use parallax to judge the distance of things all the time. Your two eyes see the world from slightly different angles, and your brain crunches the numbers to tell you how far away everything is from you.

The second half of parsec refers to an arcsecond. An arcsecond is a part of a measurement of an angle. You know that a circle is 360 degrees, right? Imagine you're standing with a full view of the horizon around you. If you sliced the horizon into 360 degrees, each degree would be about twice the width of the full moon. Divide one of those degrees into 60 pieces, and you get the width of an arcminute. Divide one of those up by 60 again, and you get an arcsecond. Putting it all together, a parsec is the parallax of one arcsecond — or, in other words, if an object in the sky moves by 1 arcsecond when the Earth moves by 1 AU, the object is 1 parsec away. To give an example of this huge distance, Proxima Centauri, the star that's our closest neighbor, is only 1.3 parsecs away. Only.

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Joanie Faletto

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