Glossary term: Parsec

Description: The parsec (pc) is a standard unit of distance measure in astronomy, defined as follows: Imagine a circle with a radius of one astronomical unit (which is the average Earth–Sun distance), which we are viewing face-on. From a distance of exactly one parsec, we will see the circle's radius subtend an angle of one arcsecond. This makes 1 pc equal to approximately 3.26 light years, and to as many astronomical units as there are arcseconds in one radian: 206,264.8 astronomical units. The practical importance of this definition is its relation to the parallax method of distance determination. For this method, one measures tiny position shifts of astronomical objects in the sky as the observer position changes by a given length. For an astronomical object at a distance of one parsec, a change in observer position by one astronomical unit (usually because the Earth has moved along its orbit between the two observations) corresponds to an apparent position shift, called the object's parallax (angle), of one arcsecond. This results in a simple relationship: The distance in parsecs is one divided by the parallax angle in arcseconds. Its direct geometrical meaning makes the parsec the professional astronomer's preferred measure of distance, more common in the literature than distances given in light years. Sirius is 2.7 pc distant from us, corresponding to a parallax of 1/2.7=0.37 arcseconds. Even Proxima Centauri, the closest star apart from our Sun, has a parallax of less than 1 arcsecond.

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