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### Parsec (pc)

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Date: 10/26/2001 at 09:30:38
From: Peter Mott
Subject: km ->pc

Dear Dr.Math,

I recently bought a Casio fx-991MS calculator. One of the unit
conversions it does is pc to km and km to pc. Would you please explain
to me what "pc" means in this context?

Thank you very much.
Regards,
Peter
```

```
Date: 10/26/2001 at 12:36:14
From: Doctor Sarah
Subject: Re: km ->pc

Hi Peter - thanks for writing to Dr. Math.

From Russ Rowlett's _How Many? A Dictionary of Units of Measurement_

http://www.unc.edu/~rowlett/units/dictP.html

parsec (pc) -

a non-metric unit of distance used in astronomy. As the Earth makes
its orbit around the Sun, nearby stars appear to shift their
positions in relation to the background of distant stars. The
shift, called the parallax of the star, is very small, less than
one arcsecond even for the nearest stars. If it can be measured,
however, then simple trigonometry can be used to find the distance
from the Earth to the star. One parsec is the distance at which a
star appears to shift its position by one arcsecond over the course
the time (about 3 months) in which the Earth moves a distance of
one astronomical unit (au) in the direction perpendicular to the
direction to the star. Using this unit makes it easy to compute
distances: the distance to a star, in parsecs, is simply one
divided by the amount of the parallax. If the parallax is 0.01
seconds, the distance is 100 parsecs. The length of
a parsec, divided by one astronomical unit (the radius of the
Earth's orbit) is the trigonometric function of the parallax called
the cotangent; from this relation we can compute that one parsec
equals 206 264.8 au. This is the same as 3.261 631 light years,
30.856 78 petameters (30.856 78 x 1012 kilometers), or about
19 173 510 000 000 miles.

- Doctor Sarah, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Definitions
High School Physics/Chemistry
Middle School Definitions
Middle School Terms/Units of Measurement

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