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/
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