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'We Don't Planet' Episode 10: The Parsec

The parsec is one of the most commonly used distance measurements in astronomy — even more so than the light-year. The word "parsec" itself is a portmanteau of "parallax" and "arcsecond." The method of parallax is used to measure distances to nearby stars. As the Earth orbits around the sun, stars will appear to wiggle back and forth. You can replicate this by holding an object (say, a pencil) close to your face while alternately closing each eye. You'll see the pencil wiggle left and right relative to objects in the background, and as you move the pencil farther away, the wiggle will become less pronounced. 

It's a very simple and direct method that requires only careful observations and a little bit of trigonometry to work out distances. Astronomers have successfully used it on tens of thousands of stars, and use these measurements to build the cosmic distance ladder to determine distances to even farther locations in the universe.

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The amount of stellar wiggle in the parallax method is measured in angles — degrees, arcminutes, and arcseconds, and these angles can used to define distances. Thus the parsec: the distance required to observe a parallax displacement of one arcsecond when viewing the object 6 months apart (i.e., when the Earth is at opposite sides of the sun). 

That works out to about 3.26 light-years. In this system, the nearest star to the sun, Proxima Centauri, is 1.3 parsecs away. Greater distances are given the traditional Greek prefixes: 1,000 parsecs is a kiloparsec (kpc), one million a megaparsec (Mpc), etc. For example, the Milky Way galaxy is about 30 kpc across, the Andromeda Galaxy is about 0.78 Mpc away, and the entire observable universe is about 28.5 gigaparsecs from end to end.

"We Don't Planet" is hosted by Ohio State University astrophysicist and COSI chief scientist Paul Sutter with undergraduate student Anna Voelker. Produced by Doug Dangler, ASC Technology Services. Supported by The Ohio State University Department of Astronomy and Center for Cosmology and AstroParticle Physics. You can follow Paul on Twitter and Facebook. Original article on

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