parsec

(pc or psc)

   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. This shift, called the parallax of the star, is very small, less than one arcsecond even for the nearest stars. One parsec is the distance at which a star would appear to shift its position by one arcsecond during the time 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 parallax, in arcseconds. If the parallax is 0.01 arcsecond, the distance is 100 parsecs. One parsec divided by one astronomical unit (the length of the semimajor axis of the Earth's elliptical orbit) is the trigonometric function of 0.01 arcsecond called the cotangent; from this relation we can compute that one parsec equals 206 264.8 au. This is the same as about 3.261 56 light years, 30.856 78 petameters (30.856 78 x 10¹² kilometers), or 19 173 510 000 000 miles.