B What is a parsec and how is it measured?

[dnl117]

TL;DR Summary

I'm quite the amateur and i couldn't find my answer in the web so maybe someone here could help me with understanding of parsecs

So i was trying to find out what parsec actually is, but i just can't understand one thing, which is what is the object parallax is measured from? Like i know it's the star A being compared to background made of even further stars B, but because if the object (star A) were to be closer or further away from earth wouldn't it change the angle and therefore the parallax? So how did they come up with the one exact distance of parsec? P.S. English isn't my first language so sorry for hard read

 

[Halc]

If the star is exactly one parsec away, it wobbles back and forth 1 arcsecond in each direction from the middle. If a star is closer, the parallax is more, and it is less if the star is further away.
So sin(1 arcsecond) is 1 AU/1 parsec

 

[fresh_42]

It is the distance from which the mean radius of the earth's orbit (= 1 AU, astronomical unit), i.e. the mean distance between the sun and the earth, appears at an angle of one arc second, and corresponds to around 206,265 astronomical units or around 3.26 light-years or about 30.9 trillion kilometers; $3.0857\cdot 10^{16}\,\operatorname{m}.$

 

[phinds]

i couldn't find my answer in the web

??? --- I find that astounding, since a simple web search of "what is a parsec" gives exactly the answer already given above.

https://en.wikipedia.org/wiki/Parsec

 

[dnl117]

??? --- I find that astounding, since a simple web search of "what is a parsec" gives exactly the answer already given above.

https://en.wikipedia.org/wiki/Parsec

Yeahh, i should've just gone with it, but instead i asked wrong question in Google and it gave me unclear answer which pointed me in the wrong direction and completely made me misunderstand the topic.

 

[Vanadium 50]

Because the Earth's orbit is an ellipse, not a circle, a "parallax of one arc-second" is not adequately defined. So a parsec is now defined as exactly 96939420213600000/π meters.

The more interesting question is "what is a light year" which is now defined as 1/3.26 of a parsec. That is the distance light travels in 1.0005 years.

 

[berkeman]

That is the distance light travels in 1.0005 years.

Oops. We should probably rename it a light year-ish...

 

[fresh_42]

Oops. We should probably rename it a light year-ish...

Year itself isn't well-defined so it's automatically already -ish.