555px-Stellarparallax parsec1.svg

Parsec Definition diagram The apparent motion of a nearby star is a small ellipse in the sky relative to background stars over the period of a year. The viewpoint is defined as the Earth. The angle representing major axis radius of the elliptical path is the parallax angle. A parsec is defined as the distance to an object whose parallax angle = 1 arc second.

A parsec (symbol: pc) is a unit of length that is used in measuring large distances of astronomical objects outside the Solar System. A parsec was defined as the distance at which one astronomical unit subtends an angle of one arcsecond.[1] It was redefined in 2015 to exactly 648,000/π astronomical units. One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun.[2]


  1. "Cosmic Distance Scales - The Milky Way". Retrieved 24 September 2014. 
  2. Benedict, G. F.. "Astrometric Stability and Precision of Fine Guidance Sensor #3: The Parallax and Proper Motion of Proxima Centauri" (PDF). Proceedings of the HST Calibration Workshop. pp. 380–384. Retrieved 11 July 2007.