It’s about distance, not time

Happy International Astronomy Day.

It has been cloudy here in the northwest, so it is hard to do astronomy outside. Instead, I thought I would define two astronomical words that often confuse people, and with good reason. These words are “light-year” and “parsec.”

Both of these words have a time word in them, so logic would say they measure time. However, they are actually measurements of distance. Yep, Han Solo got it wrong (more accurately, George Lucas got it wrong), and no amount of explaining is going to fix it. However, as I just said, it’s an innocent mistake. Anyone could think the same thing.

So what do these words mean? A light-year is easy. It is the DISTANCE that light travels in one Earth year, and it is a constant (5.879 trillion miles.) You know it is a measurement of distance because it is defined in miles or kilometers.

A parsec is harder. In a nutshell, a parsec is about 3.26 light-years. It is used to measure the distance to stars and other faraway objects. The cool part is that not only is a parsec a measurement of distance, but its name also explains how that measurement is made. A parsec is the distance to an object having a parallax of one second, as seen from points separated by one astronomical unit. Get it? A parallax of one second – a parsec.

Defining a parsec is easy, understanding that definition is harder. Instead of reinventing the wheel, I found a cool video by a science blogger I follow. He does a good job of explaining a parsec without requiring you have a physics Ph.D. If you like illustrations, you can check out this video, too.

In honor of International Astronomy Day, do some stargazing if you have clear skies. If you don’t, spread the word that light-year and parsec are measurements of distance, and Han shot first.