Parsecs | Universe Today

Written by John Carl Villanueva

One lightyear is roughly equivalent to 0.3066 parsecs. While the general public is perhaps more familiar with lightyears than parsecs due to the influence of pop culture, it is actually easier to express the distance between a star and the Sun (and us) in terms of the latter.

How is that so?

First, let's start with just the term 'parsec'. It's actually a combination of two words – parallax and arcsecond. Apparently, we'll need to define both in order to better understand what a parsec is.

I'll need your participation as I strive to make you understand the term 'parallax'.

Ready? Alright, hold your thumb right in front of your nose and shut your left eye. At the same time, stare at a single object that's in front of you. Now, open your left eye and shut your right eye while still staring at the same object. Notice how your thumb appears to have moved relative to the object?

Next, extend you hand out front and repeat the same procedure. You should be able to observe that the thumb now appears to have moved a shorter distance relative to the object. This apparent movement is called parallax. The parallax is larger when your thumb is near you (the observer) and smaller when your thumb is far.

Astronomers have the same observations when they view a star's position relative to stars in the background when they (the astronomers) move from one location to another (say, when they view the same star first in January and then in June – the earth is going to be at opposites sides of its orbit at these months).

Thus, the farther the star is, the lesser will its parallax be. This parallax is usually expressed in arcsecond, a unit of angle. Basically when the parallax angle is one arcsecond, then the distance of the observed star from us is defined to be one parsec.

Based on the observations we made earlier, we can conclude that if a parallax angle is more than one arcsecond, then the distance of the star (in parsecs) would be less than one parsec, i.e., it should be nearer. But if the parallax angle is less than one arcsecond, the distance of the star would be more than one parsec.

Without going into the math, you can easily see the relationship between parsecs and the parallax angle. Thus, it is easier to express stellar distances in terms of parsecs.

Here's a link from Universe Today that will lead you to more detailed discussions regarding parallax and parsecs.

Still find it confusing? How about a related article from NASA instead?

Here are two episodes at Astronomy Cast that you might want to check out as well:
Large Scale Structure of the Universe
Infrared Astronomy

Filed under: Astronomy