A parsec is a unit of distance, often used by astronomers as an alternative to the light year, just as kilometers can be used as an alternative to miles. Sci-fi franchises such as Star Wars have been known to misuse the word “parsec” and incorrectly describe it as a measure of time or speed.
In fact, a parsec is approximately 3.26 light-years, or almost 19 trillion miles (31 trillion km), according to the California Institute of Technology. (opens in new tab) (Kaltech).
The term parsec is a combination of “parallax” and “arcsecond,” derived from the use of triangulation in measuring the distance between two stars.
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Arc seconds and the parallex effect
astronomers Use arcseconds to measure very small angles, with 3,600 seconds making up a degree, just as there are 3,600 seconds in an hour. These small angles help astronomers measure large distances using something called the parallax effect.
If you hold a pencil up at arm’s length and alternately close your left and right eyes, you’ll notice that the pencil appears to move left and right relative to objects that are farther away, even when you’re holding it completely stationary.
This is the parallax effect and it happens because the angular direction to the pen is slightly different when seen by your left and right eyes. If you could measure this difference in angle, knowing the distance between your eyes you could calculate the distance to the pencil.
The same principle allows astronomers to measure the distance to nearby stars. They photograph a piece of sky containing the star they are interested in and other more distant objects such as galaxies.
Six months later, when Earth is on the other side of the sun, they take another photo of the same piece of sky, according to NASA. The star appears to have moved a small angular distance relative to the background objects. If we measure this angle and then bisect it (because we have two equal and opposite offsets relative to the sun) we get the star’s parallax.
So that’s where the parsec comes from: it’s the hypothetical distance at which a star would exhibit a parallax of exactly one second. In fact, true stellar parallaxes are smaller than this, meaning their distances are always greater than one parsec.
parsecs against light year
As logical as the definition of a parsec is, it will probably still seem unnecessarily complicated to most people. In contrast, the light year is much easier to understand. It is simply the distance light travels in a year and has been used since at least 1838.
The light year has uses even beyond a simple measurement, for it tells us that if we observe an object X light years away, we see it as if it was X years in the past. So why would anyone use parsecs instead?
The answer seems to be that when astronomers first started measuring stellar distances using the parallax method, they simply presented their results in terms of “a parallax of X seconds” instead of converting them to light-years.
Then, around 1913, Herbert Hall Turner had the idea of shortening this to parsec – and the name stuck even as other, non-parallax-based methods of measuring the distance of stars were developed. Today the International Astronomical Union (opens in new tab) recommends the use of parsecs across light years in scientific papers, although the latter is still very common in the vernacular.
The Royal Astronomical Society (opens in new tab) has information and articles on parsecs on its website. The International Astronomical Union (opens in new tab) also contains a wealth of material on the subject.