Parsec

The parsec (symbol: pc) is a unit of length used in astronomy, equal to about 30.9 trillion kilometres (19.2 trillion miles). In astronomical terms, it is equal to 3.26 light-years, and in scientific terms it is equal to 3.09×1013 kilometres (1.92×1013 miles).

The name parsec is "an abbreviated form of 'a distance corresponding to a parallax of one arc second'."[1] It was coined in 1913 at the suggestion of British astronomer Herbert Hall Turner . A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond (1⁄3,600 of a degree). In other words, if a straight line were drawn from the object to the Earth, and another line drawn from the object to the Sun, if the angle formed between the two lines is exactly one arcsecond, then the object's distance would be exactly one parsec.

The angle can be measured by observing the object's spot in the sky. It becomes quite convenient for astronomers to determine the object's distance by simply measuring its position in the sky at 6-month intervals (when the Earth is at opposite ends of its orbit around the Sun). In this case, the object's distance in parsecs is numerically (though not dimensionally) equal to the reciprocal of the number of arcseconds by which its position changes .[2] The less an object moves, the farther it is from the Sun, and vice versa.