What is a parsec and how is it measured?

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In summary, the parsec 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.
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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 🤠
 
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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
 
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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}.##
 
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dnl117 said:
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
 
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phinds said:
??? --- 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.
 
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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.
 
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Vanadium 50 said:
That is the distance light travels in 1.0005 years.
Oops. We should probably rename it a light year-ish... :wink:
 
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berkeman said:
Oops. We should probably rename it a light year-ish... :wink:
Year itself isn't well-defined so it's automatically already -ish.
 
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FAQ: What is a parsec and how is it measured?

What is a parsec?

A parsec is a unit of distance used in astronomy to measure the vast spaces between celestial objects. It stands for "parallax of one arcsecond" and is equivalent to approximately 3.26 light-years or about 3.086 x 10^13 kilometers.

How is a parsec measured?

A parsec is measured using the method of parallax, which involves observing the apparent motion of a nearby star against the background of more distant stars as Earth orbits the Sun. By measuring the angle of this apparent shift (parallax angle) and knowing the distance Earth travels in its orbit, astronomers can calculate the distance to the star using trigonometry.

Why do astronomers use parsecs instead of light-years?

Astronomers use parsecs because they are directly related to the measurement technique of stellar parallax, making it a convenient unit when calculating distances based on observational data. While light-years are more commonly used in popular science, parsecs are more practical for professional astronomical calculations.

Can you provide an example of how a parsec is calculated?

Suppose a star exhibits a parallax angle of 1 arcsecond (1/3600 of a degree). Using the parallax formula, the distance to the star (in parsecs) is the reciprocal of the parallax angle in arcseconds. Therefore, if the parallax angle is 1 arcsecond, the distance is 1 parsec. For smaller angles, the distance increases; for example, a parallax angle of 0.5 arcseconds corresponds to a distance of 2 parsecs.

What are some common objects measured in parsecs?

Common objects measured in parsecs include stars within our galaxy, other galaxies, and various astronomical phenomena such as star clusters and nebulae. For instance, the nearest star to the Sun, Proxima Centauri, is about 1.3 parsecs away, while the Andromeda Galaxy is approximately 780,000 parsecs away.

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