Parsecs Part One: Background

If you've ever seen the original Star Wars, you'll no doubt remember Han Solo's famous boast about the Millennium Falcon, "It's the ship that made the Kessel Run in less than twelve parsecs." This usage turns out to be incorrect for two reasons.

But before we get into that, some background information is needed. The name "parsec" is unfortunate, as it sounds like a fancy science-fiction term for "part of a second". As it turns out, parsecs do relate to seconds, but not seconds of time. It refers to seconds of arc.

Now lets talk about what a 'second of arc' is. Hold your hand out as far as you can and look at your pinky finger. At arm's length, your pinky is appears to be about 1 degree in width. There are 360 degrees in a circle, so if you copied your pinky 360 times you could make a loop around yourself at arm's length.

Each degree is divided into "minutes of arc", or arc-minutes. There are 60 arc-minutes in1 degree of arc. So looking back at your pinky, an arcminute is 1/60 of that width.

Finally, each arcminute is further divided into "seconds of arc", or arc-seconds. There are 60 arcseconds in an arcminute. At arm's length, that means an arcsecond is a mind-boggling 1/3600 the width of your pinky.

Now that we have our background information, we can discuss what a parsec actualy is here.