Wednesday, November 16, 2011

The term parsec, coined by Oxford professor Herbert Hall Turner

A parsec is a unit of length used to express astronomical distance. According to the Oxford Dictionary of Physics, a parsec is the distance at which the mean radius of the earth's orbit subtends an angle of one second of arc [1]. A more illustrative definition is given by Neil deGrasse Tyson in a footnote [2]:
A unit of distance in astronomy equal to 3.26 light-years, itself equal to about 19 trillion miles and derived from the distance a star would have to be for it to exhibit a parallax angle of 1 second of arc (hence par-sec) against the background stars as Earth orbits from one side of the Sun to the other [text coloring by post author].
Tyson further tells us that the Oxford professor and former astronomer royal Herbert Hall Turner coined the term parsec. In 1930, Turner also played his part in promoting the name Pluto for a cosmic object discovered by Clyde Tombaugh and at first referred to as Planet X.    

The Google calculator result for 1 parsec:

1 Parsec = 3.08568025 × 1016 meters

Still not satisfied? Find some nice video animations and lectures at (for example Parsec definition and What is a Parsec?).

Keywords: physics, astronomy, units, history

References and more to explore
[1] Alan Isaacs: Oxford Dictionary of Physics. Oxford University Press, Oxford and New York, 2003 (reissued fourth edition). 
[2] Neil deGrasse Tyson: The Pluto Files. W. W. Norton & Company, New York and London, 2009; page 9, 11.

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