parsec

A parsec is a unit of distance, e.g., to stars, that is generally used by astronomers rather than light-years. It was developed for parallax measurements to stars, being distance corresponding to a parallax of one arcsecond, i.e., if a star shows a parallax of one arcsecond, it is one parsec from the Sun. A parsec is approximately:

• 3.261564 light-years (effectively, the number of years for light to travel one parsec).
• 3.08567758 × 10¹⁶ meters.
• 19173512 million miles.
• 206264.806 AU.

If a star appears to move back and forth two arcseconds over the course of a year as the Earth moves to and from a spot 2 AU away (1 arcsecond parallax angle), the star would be 1 parsec distant. If the apparent movement were half that, then the star would be two parsecs distant and if a third of that, three parsecs distant.

Note that the International Astronomical Union (IAU) has quoted a parsec as being 64800/π AU (64800/π is the number of radians in an arcsecond and use of that factor is a small angle approximation of the tangent of that angle). An IAU resolution has defined the AU to be 149597870700 meters, which is a nominal value of the average distance between the Earth and Sun. These allow distances cited in parsecs to be as exact as SI distance measurements, and stable regarding any changes in or improved determinations of Earth orbit.