parsec

Parsec is a portmanteau coined by British astronomer Herbert Hall Turner in 1913, contracting parallax and arcsecond. An arcsecond is one 3,600th of a degree — a unit of angular measurement so small that a human hair held at arm's length subtends roughly ten arcseconds. A parsec is defined as the distance at which a star would have a parallax of exactly one arcsecond: that is, if Earth's orbital motion — a baseline of one astronomical unit — were observed from that star, it would span one arcsecond of angle. The unit emerged from observational practice: stellar parallaxes are measured in arcseconds, and dividing an astronomical unit by the tangent of an arcsecond gives a distance. Turner's portmanteau embedded the measurement method directly in the unit's name.

The parsec is approximately 3.086 × 10¹³ kilometers, or 3.262 light-years, or about 206,265 astronomical units. No star is close enough for a parallax of exactly one arcsecond — even the nearest star system, Alpha Centauri, has a parallax of 0.747 arcseconds, placing it at 1.34 parsecs. Most stars in the Milky Way are tens to thousands of parsecs away; the galaxy itself is about 30,000 parsecs (30 kiloparsecs) in diameter; the nearest large galaxy, the Andromeda Galaxy, is about 0.78 megaparsecs away; and the observable universe extends to roughly 14,000 megaparsecs. Astronomers naturally prefer parsecs to light-years because parsecs derive directly from the parallax measurements that are the primary method of determining stellar distances — using light-years would require an extra conversion at every step.

The light-year is vastly more familiar to the general public than the parsec, largely because of its self-explanatory name — the distance light travels in one year, approximately 9.46 × 10¹² kilometers. The light-year entered popular astronomy writing in the nineteenth century and has been the preferred unit of popular science ever since, despite being calculationally awkward for astronomers. The parsec, by contrast, has remained almost entirely a professional term. A notable confusion: Han Solo's famous boast in Star Wars that the Millennium Falcon made the Kessel Run in 'less than twelve parsecs' treats parsecs as a unit of time rather than distance. The screenwriters almost certainly meant light-years. The scientific community has enjoyed this error for decades; the later sequel films inserted a convoluted retcon to explain it away.

The kiloparsec (1,000 parsecs) and megaparsec (1,000,000 parsecs) are the working units of galactic and extragalactic astronomy. The Hubble constant — the rate of the universe's expansion — is conventionally expressed in kilometers per second per megaparsec: the universe currently expands at roughly 70 kilometers per second for every megaparsec of distance between two points. The gigaparsec (10⁹ parsecs) appears in discussions of the observable universe's scale. Turner's 1913 portmanteau, constructed for the narrow practical purpose of naming the base unit of stellar distance measurement, has scaled upward to encompass the entire observable universe. The arcsecond's distance — the smallest angle reliably measurable in Turner's era — turned out to be a convenient ruler for everything from nearby stars to the cosmic horizon.