What is a Parsec?

by Stephanie Bender on November 14, 2013

This article was originally written in 2009, but we’ve updated it with this spiffy new video from Fraser!

A parsec is equivalent to 3.26 light years. And since a light year is the distance light travels in 1 year 9.4 trillion km, 1 parsec equals 30.8 trillion km. So, where then, does the term, “parsec”, come from? Parsec is a combination of 2 words, parallax (par) and arc second (sec).

Parallax means something looks like it changed its location because you changed yours. For example, if you stand on your porch and look across the street, you will see a house on your left and a house on your right. If you go across the street and look at the same houses from your neighbor’s backyard, they will be on the opposite sides. Did the houses move? Of course not. You changed your location. Since you are in a different place, facing a different direction, they appear to be in different places. Likewise, two different people, in two different parts of the world, might see the exact same event in the sky or outer space; yet, it might appear entirely different due to their locations.

Astronomers measure parallax by measuring how distant stars shift back and forth as the Earth travels around the Sun. Astronomers measure the position of the stars at one time of the year, when the Earth is at a position in its orbit around the Sun, and then they measure again 6 months later when the Earth is on the other side of its orbit. Nearby stars will have shifted a tiny amount compared to more distant stars, and sensitive instruments can detect the change.

Parsec diagram

Now for the second part of “parsec”: arcsecond. In this instance, we’re not referring to a measure of time. It’s a part of a measurement of angle. Imagine the horizon around you broken up into 360 slices, or degrees. Each slice is about twice the width of the full moon. An arcminute is 1/60th of a degree, and an arcsecond is 1/60th of an arcminute. So astronomers measure the size of objects, or the parallax movement of stars in degrees, arcminutes and arcseconds.

So, to put those terms together, a parsec is the parallax of one arcsecond. Just a warning, you’re going to need to dust off your trigonometry for this. If you create a triangle, where one leg is the distance between the Earth and the Sun (one astronomical unit), and the opposite angle, measured by how far the star moves in the sky, is one arcsecond, the star will be 1 parsec away, the other leg of the triangle.

For example, the closest star in the sky, Proxima Centauri, has a parallax measurement of 0.77233 arcseconds – that’s how far it shifts in the sky from when the Earth shifts its position by 1 astronomical unit. If you put this into the calculation, you determine that Proxima Centauri is 1.295 parsecs away, or 4.225 light years.

We have written many articles about the parsec for Universe Today. The article explains how the astronomical unit might need to be changed as the Sun loses mass.

Here’s an article from NASA that explains how to derive the parallax measurement.

Still doesn’t make sense? Check out his episode of Astronomy Cast where we explain various methods astronomers use to measure the Universe. Episode 10: Measuring Distance in the Universe.

Source: Wikipedia

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