Have you ever heard astronomers use the term "parsec" to describe distances in space? More importantly, have you ever heard that the Millennium Falcon can make the Kessel Run in less than 12 parsecs? What the heck does this word mean? A parsec is an astronomical term for measuring distance in space. A single parsec is approximately 30.9 trillion kilometers, or 3.26 light years. In other words, it takes photons of light over three years to travel a parsec's distance. Parsecs are used for measuring really really large distances. Here in our Solar System, we usually use kilometers or astronomical units. In the distances between stars, astronomers measure distances in parsecs, and for the vast gulfs between galaxies it's kiloparsecs, megaparsecs and gigaparsecs. It seems like a strange number, though, 3.26 light years. So where does the concept of a parsec come from? Here's the technical term: "a parsec is the distance corresponding to a parallax of one arcsecond" Need more information than that? Okay fine. Consider how the Earth travels around the Sun in its orbit. At one point in the year, it's on one side of the Sun, and then 6 months later, the Earth has traveled to the opposite point in its orbit. From our perspective here on Earth, a nearby star will appear to move back and forth a tiny little bit against the background Universe. You can get this same effect by holding up your thumb, staring down your arm, and then opening and closing your eyes. See how your thumb is moving back and forth compared to the background scenery? This is known as parallax. When a foreground object moves compared to a more distant background. By measuring the changing angles from the observer to the star and the background Universe, you can calculate the distance. Imagine a right-angle triangle, the bottom of the triangle is the distance from the Earth to the Sun, also known as an astronomical unit. The side of the triangle is the distance from the Sun to the star. And then there's the angle required to make the star appear to move 1 arc second in the sky. The sky is broken up into 360 degrees. Each degree is broken up into 60 arc minutes, and there are 60 arc seconds in an arc minute. So one arc second is 1/3600th of the sky. 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. To give you another comparison, Saturn appears to be about 14 arc seconds when it's at its most distant point in the sky, so astronomers are measuring really tiny distances. So now I hope you have a better idea of what a parsec is, and how astronomers use it to measure distances in the Universe. But what does it means to make the Kessel run in under 12 parsecs? Well, that's another episode.

• published: 14 Nov 2013
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