Date: December 18th, 2012

Title: Astronomy Word of the Week : Parsec

Podcaster: Dr. Christopher Crockett

Organization: United States Naval Observatory


Description: Astronomers don’t typically use light-years when talking about distances to stars, nebulae, and quasars. They use a unit of measurement more closely related to how cosmic distances are actually measured—and depends entirely on the size of Earth’s orbit.  The astronomy word of the week is “parsec”.

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. Please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at


You know, if I was Obi-Wan Kenobi, I probably would have looked elsewhere for passage to Alderaan after hearing this…

Han Solo: “You’ve never heard of the Millenium Falcon?”
Obi-Wan Kenobi: “Should I have?”
Han Solo: “It’s the ship that made the Kessel Run in less than 12 parsecs…”

Whoa, slow down there Han.  That might sound impressive to the average Joe.  But listeners of this podcast—and certainly Mr. Kenobi—know better.  Parsecs aren’t a measure of time.  They’re a unit of distance.

If you ever listen to astronomers talk amongst themselves, you won’t hear much talk of “light-years”.  It’s a great way to think about the scales of the Universe.  But not very useful when it comes to actually measuring those distances.

Parsecs are much more closely related to how we actually go about the business of figuring out the size of the Universe.

To find the distance to a star, we use triangulation.  In fact, you can do this right now.  Hold your finger in front of your face, focus on something in the distance, and close one eye. Now switch which eye is open.  If you alternate eyes, you’ll notice your finger appears to dance back and forth in front of your face.  The motion is, of course, an illusion.  Each eye sees your finger from a different angle so it’s location relative to stuff in the background is going to look different.

This apparent shift is called “parallax” from the Greek word for “alteration”.

Rather than blink our eyes, astronomers move the Earth.  Or rather, we let the Earth move around the Sun.  If we observe a star in January and then look at it again in July, the Earth will have gone half way around its orbit.  We’re looking at the star from two locations approximately 300 million kilometers apart.  If the star is reasonably close, it will appear to move ever so slightly.  The parallax angle, combined with the size of Earth’s orbit, lets astronomers calculate the distance to the star.

These angles are miniscule.  Too small for degrees to be a practical unit of measurement.  Parallax angles are typically measured in arcseconds.  There are 3600 arcseconds in one degree.  To provide some perspective: one arcsecond is equivalent to the width of an average human hair seen from 20 meters away.

And here’s how we arrive at parsecs as a unit of distance.  One parsec is the distance to an object who’s parallax angle is one arcsecond.

The term first appeared in a 1913 paper by English astronomer Frank Watson Dyson. He wrote: “There is need for a name for this unit of distance. Mr. Charlier has suggested Siriometer, but if the violence to the Greek language can be overlooked, the word Astron might be adopted.  Professor Turner suggests Parsec, which may be taken as an abbreviated form of “a distance corresponding to a parallax of one second.””

Parallax plus second equals parsec!

The Professor Turner he refers to is British astronomer Herbert Hall Turner.  He was the Chief Assistant at the Greenwich Observatory in the late 1800s and went on to become Director of the Observatory at Oxford University.  And it was he who introduced the astronomy community to the word “parsec”.

The idea stuck.   If you see a star with 1/2 arcsecond of parallax, it is two parsecs away.  At 1/3 arcseond, three parsecs.  And at 1/4 arcsecond, four parsecs.  And so on.  You can also turn this around.  If you see a nebula that is one arcsecond in diameter, then it’s actual size is one astronomical unit (AU)–the radius of Earth’s orbit.  A five arcsecond diameter cloud is five AU, and so on.

Basically, it stuck around because it makes the math easier!

One parsec is approximately 30 trillion kilometers or 19 trillion miles.  That’s a bit over three light-years.  The Voyager 1 probe, launched in 1977, is the most distant manmade object from Earth.  It is a mere six ten-thousandths of a parsec away.  The nearest star to the Sun, a small red dwarf named Proxima Centauri, is just over one parsec from us.  That is actually fairly typical in our neck of the Galaxy: one star for every cubic parsec. But not everywhere.  In the cores of globular clusters, the density can reach well over a hundred stars per cubic parsec!

The center of the galaxy lies just over 8000 parsecs from us in the direction of the constellation Sagittarius.  The Andromeda Galaxy, the closest spiral galaxy to our own, is nearly 800 kiloparsecs away.

At larger scales, astronomers start to talk of megaparsecs and even gigaparsecs.  That’s one million and one billion parsecs, respectively.  These are generally reserved for the largest structures in existence.  The Virgo Galactic Cluster, a conglomeration of thousands of galaxies which our own Local Group is falling towards, lies 16 megaparsecs from home.  It would take 54 million years to reach it traveling at the speed of light.

The edge of the visible universe–the cosmic horizon–is the limit of our vision.  And it is fourteen gigaparsecs away.  That’s 46 billion light-years.  The universe isn’t old enough for light beyond this region to have yet reached us. We don’t know what lies beyond.  More of the same?  Or is our bubble of existence different from the rest of creation?  We probably won’t know until we get out there.

And with Voyager 1, we are 40 quadrillionths of the way there!

End of podcast:

365 Days of Astronomy
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