What is a parsec?

[ilan]

What is a parsec?
Christopher Crockett in SPACE | September 11, 2018

Why do professional astronomers speak of distances in the universe not in terms of light-years, but in terms of parsecs, a distance of 3.26 light-years? Explanation here.​

If you ever heard professional astronomers talking among themselves, you wouldn’t hear much talk of light-years. The concept of a light-year – the distance light travels in a single earthly year, or about 6 trillion miles (nearly 10 million km) – is a great way to think about distance scales in the universe. But light-years aren’t as useful as parsecs when it comes to measuring those distances. A parsec – a unit of distance equal to about 19 trillion miles (more than 30 trillion km) – is more closely related to how astronomers go about the business of figuring out the size of the universe.

To find the distance to a nearby star, astronomers use triangulation. You can try it for yourself, right now. Hold your finger in front of your face, focus on something in the distance, and close first one eye, then the other eye. As you alternate eyes, you’ll notice your finger appears to dance back and forth in front of your face. The motion is, of course, an illusion. Your finger isn’t moving. Each eye sees your finger from a slightly different angle. So the finger’s location, relative to stuff in the background, looks different. This apparent shift is called parallax, from a Greek word meaning alternation.

If you measure the angle over which your finger appears to move, you can figure out how far your finger is from your face. Likewise, astronomers measure angles to find the distances to stars. Rather than blink their eyes, however, astronomers move the Earth.

Or rather, we use the fact that Earth moves around the sun.

Ready for a definition of parsec? Here it is. One parsec is the distance to an object whose parallax angle is one arcsecond. Don’t be thrown by the terms parallax angle, and arcsecond. Keep reading, and we’ll explain.

The video below from Las Cumbres Observatory does a good job explaining what a parallax angle is.

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Or look at the diagram below. It illustrates the definition of parallax angle, and also of the word parsec:

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Yeah, I know...MATH! But even if you don't quite get it (or remember how to do the calculations), you'll probably remember the term and that it's how distance is calculated in astronomical circles. - ilan