Just as kilometers can be used as an alternative to miles, a parsec can be used as an alternative to the light year. "Star Wars" has used the word "parsec" wrongly to describe it as a measurement of time or speed.
The California Institute of technology says that one parsec is approximately 19 trillion miles.
"parallax" and "arcsecond" are used to measure the distance between two stars.
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Astronomers use 3,600 seconds to measure very small angles, just as there are 3,600 seconds in an hour. Astronomers use the parallax effect to measure distances.
If you hold a pencil up at arm's length and close your eyes, you will notice that it moves left and right, even if you keep it stationary.
When you look at the pencil with your left and right eyes, there is a slight difference in the direction of the pencil. Knowing the distance between your eyes will allow you to calculate the distance to the pencil.
Calculating the distance to nearby stars is possible thanks to the same principle. They take a picture of a patch of sky containing a star and other distant objects.
When the Earth is on the other side of the sun, they take a picture of the same area. The star appears to have moved through a small distance. The star has a parallax because we have two equal and opposite offsets.
It is the distance at which a star would show a parallax of one second. The distances of real stellar parallaxes are larger than that.
As logical as the definition is, it is still likely to be confusing to most people. The light-year is simpler to comprehend. It's the distance that light travels in a year, and it's been used for over 200 years.
The light-year tells us that when we see an object X light-years away, it is the same object we saw in the past. Why would anyone use Parsecs?
The answer seems to be that when astronomy began measuring stellar distances using the parallax method, they simply presented their results in terms of a "parasaic of X seconds" rather than converting to light years.
The name stuck even after other, non-parallax-based, methods of measuring stellar distance were developed. The International Astronomical Union recommends the use of parsecs over light-years in scientific papers.
The Royal Astronomical Society (opens in new tab) has information and articles on its site relating to parsecs. The International Astronomical Union (opens in new tab) similarly contains a wealth of material on the subject.
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