# Parsec Question.

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#### ccart3r

##### Guest
I'm trying to understand Parsec and saw a question and wondering if I did it right.

The question being, If a meteor is traveling to earth at 10 parsecs, When would it reach earth?

Now I don't know if I did it right but the answer I came up with is 32.6 years. Is this correct and if not what is and how did you come up with it?

Thanks, sorry if this is the wrong section.

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#### SpeedFreek

##### Guest
Hi there, welcome to the SDC (space dot com) forums!

A parsec is only a unit of distance, equal to around 3.26 light-years.

Unfortunately, your question is not answerable, because you haven't said how fast your meteor is moving towards us, only how far away it is - 10 parsecs, or 32.6 light years.

If your meteor were travelling at the speed of light (which is impossible), it would indeed take 32.6 years to reach Earth.

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#### ramparts

##### Guest
No!! Parsec is a unit of time. Han Solo said so - the Millenium Falcon made the Kessel Run in less than twelve parsecs.

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#### ccart3r

##### Guest
Ah yes, thank you. The question didn't include a speed, so I did it as if it were traveling at the speed of time, hence how i came up with 32.6 years. That's why I was confused, I wasn't sure if they forgot to mention the actual speed or not.

now say the meteor was traveling at say 100,000 mph. How would you go about answering the question then?

Thanks, and glad to be here.

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#### vogon13

##### Guest
We charge \$50 for homework questions

Paypal accepted.

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#### ccart3r

##### Guest
That would make sense if I were in school, Seeing how I'm not. And I am just interested in parsecs. and learning in general. If you don't like 100,000 then choice another speed, I'm just interested in learning how to use parsecs and how to correctly figure it out.

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#### ramparts

##### Guest
Parsecs are the same as any other unit of length, so you "use them" the same way. The easiest way to do unit conversions these days is through Google or Wolfram Alpha (wolframalpha.com). Go into either and type in something like:

1 parsec in meters

And you'll see how many meters a parsec is equal to. If you want to answer the question of how long something travelling at 100,000 miles per hour will take to reach the Earth starting from 10 parsecs away, type in smething like:

10 parsecs / (100,000 miles per hour)

(The answer is roughly 218731 years.)

Now, if you want to get a better feel for what a parsec means, you'd never see a meteor starting from 10 parsecs away - the nearest stars are only a couple of parsecs away, so the solar system (which contains just about all the meteors you're likely to see) is way smaller than that. Pluto is only about 0.0002 parsecs away. The edge of the Oort Cloud, which is believed to contain most of the solar system's comets, etc., is thought to have an edge of only about a quarter of a parsec. The distance between the Earth and the Sun is 5e-6 parsecs - that's 0.00005 parsecs

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#### ccart3r

##### Guest
ramparts":3v10cg5w said:
Parsecs are the same as any other unit of length, so you "use them" the same way. The easiest way to do unit conversions these days is through Google or Wolfram Alpha (wolframalpha.com). Go into either and type in something like:

1 parsec in meters

And you'll see how many meters a parsec is equal to. If you want to answer the question of how long something travelling at 100,000 miles per hour will take to reach the Earth starting from 10 parsecs away, type in smething like:

10 parsecs / (100,000 miles per hour)

(The answer is roughly 218731 years.)

Now, if you want to get a better feel for what a parsec means, you'd never see a meteor starting from 10 parsecs away - the nearest stars are only a couple of parsecs away, so the solar system (which contains just about all the meteors you're likely to see) is way smaller than that. Pluto is only about 0.0002 parsecs away. The edge of the Oort Cloud, which is believed to contain most of the solar system's comets, etc., is thought to have an edge of only about a quarter of a parsec. The distance between the Earth and the Sun is 5e-6 parsecs - that's 0.00005 parsecs
That's very interesting, Thank you for all the information.

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#### kg

##### Guest
ccart3r":bsjs0v7j said:
I'm trying to understand Parsec and saw a question and wondering if I did it right....
The Parsec came from 19th century astronomers who were trying to find the distances to the nearest stars.

"The parsec ("parallax of one arcsecond", symbol pc) is a unit of length, equal to just under 31 trillion kilometres (about 19 trillion miles), or about 3.26 light-years." http://en.wikipedia.org/wiki/Parsec

Here is a more modern survey of distances to stars. Prity much the same technique but done much more accuratly.
http://en.wikipedia.org/wiki/Hipparcos

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#### MeteorWayne

##### Guest
This topic really doesn't belong in Physics, so I'll move it to Ask the Astronomer
MW

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#### matthewota

##### Guest
Another thing about parsecs. It is a unit of measure that has fallen out of use by modern astronomers. I was taught in college to measure cosmic distances in Light Years instead of parsecs.

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#### ramparts

##### Guest
matthewota":34e5494k said:
Another thing about parsecs. It is a unit of measure that has fallen out of use by modern astronomers. I was taught in college to measure cosmic distances in Light Years instead of parsecs.
That is completely untrue Your college professor may prefer light years. He probably just prefers it for his non-major classes. Almost all modern astronomy research uses parsecs (or kiloparsecs, megaparsecs, etc.).

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#### SpaceTas

##### Guest
Distances to close by stars can be measured directly, by taking a series of images of the star and measuring it's position relative to far more distant stars. The near-by star will trace a little ellipse on the sky as the Earth moves around it's orbit. The angle subtended by this ellipse is the parallelactic angle (=parallax). A star at 1 parsec has a parallax of 1 arc second. This is a small angle it is 1/3600 of a degree (the moon is 1/2 deg in apparent diameter). You can turn the definition of the parsec around: 1 parsec is the distance at which the Earth orbit subtends an angle of 1 arc sec.

So from the nearest star the earth (proxima Centauri is just over a parsec away) never appears more than 1/2 an arc sec from the Sun.

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#### aphh

##### Guest
Also, it is very easy to get the distance in parsecs, when you know the parallax.

Since parallax of 1 arcsecond equals distance of 1 parsec, parallax of less than 1 arcsecond means the distance in parsec must be inverse of the parallax.

Parallax of 0.125 arcsecond = 1 / 0.125 parsec = 8 pc.

This is what Han Solo actually meant in Star Wars, when he said Millenium Falcon made the Kessel run of 12 parsec. Probably meant they could do that long run without refueling.

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