| Miles (mi) | Parsecs (pc) |
|---|---|
| 1 Mile | 5.21552870479 × 10-14 pc |
| 2 Miles | 1.04310574096 × 10-13 pc |
| 3 Miles | 1.56465861144 × 10-13 pc |
| 4 Miles | 2.08621148192 × 10-13 pc |
| 5 Miles | 2.6077643524 × 10-13 pc |
| 10 Miles | 5.21552870479 × 10-13 pc |
| 20 Miles | 1.04310574096 × 10-12 pc |
| 25 Miles | 1.3038821762 × 10-12 pc |
| 50 Miles | 2.6077643524 × 10-12 pc |
| 100 Miles | 5.21552870479 × 10-12 pc |
| Reference | Miles (mi) | Parsecs (pc) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.000184547 mi | 9.62511 × 10-18 pc |
| Average adult human height | 0.00105633 mi | 5.50932 × 10-17 pc |
| A football pitch (length) | 0.065244 mi | 3.40282 × 10-15 pc |
| A marathon | 26.2188 mi | 1.36745 × 10-12 pc |
| Height of Mount Everest | 5.49851 mi | 2.86777 × 10-13 pc |
The mile is a unit of length equal to 5280 feet, 1760 yards, or exactly 1609.344 metres. The symbol is mi. It is the principal unit of road distance in the United States and the United Kingdom.
Its name comes from the Roman mille passus, a thousand paces, where a pace counted two steps. That unit measured about 1479 metres, appreciably shorter than the modern mile. The English mile grew to its present length because it was reconciled with the furlong, an older agricultural measure. Making the mile eight furlongs required 5280 feet, and Parliament fixed that figure by statute in 1593, which is why the unit is sometimes called the statute mile to distinguish it from other historical miles.
Those alternatives were numerous. The Scots mile, the Irish mile and various German and Scandinavian miles all differed, some substantially. The Swedish mil, still used colloquially, is ten kilometres.
Road signs, speed limits and vehicle odometers use the mile in the United States, the United Kingdom, Liberia and Myanmar, and speeds are given in miles per hour. The United Kingdom's position is inconsistent by design: distances and speeds are imperial while most other official measurement is metric, an arrangement that has survived repeated proposals for change.
Athletics keeps the mile as the one non-metric distance with continuing international standing. World records are ratified over the mile alongside metric distances, largely because of the historical weight of the four-minute barrier broken by Roger Bannister in 1954. The current record stands under 3 minutes 44 seconds.
Fuel economy in the United States is expressed as miles per gallon, and aviation and shipping use the separate nautical mile, which is longer at 1852 metres and unrelated in derivation.
The map of the United States is drawn on it. The Public Land Survey, begun in 1785, divided the western territories into townships six miles square, each containing thirty-six sections of one square mile, and each section of 640 acres subdividing neatly into quarters and quarter-quarters. Roads followed the section lines, which is why so much of the American Midwest is a grid of straight routes one mile apart, and why farms there are described in quarter sections rather than in acres. Anyone flying over Iowa or Kansas is looking at the mile drawn across an entire landscape, two centuries after the surveyors laid it out.
One mile equals 1760 yards, 5280 feet, 8 furlongs, or 1609.344 metres. A five-kilometre race is about 3.107 miles.
The parsec is a unit of distance equal to about 3.0857 × 1016 metres, or 3.26 light years. The symbol is pc. It is the standard distance unit in professional astronomy, used in preference to the light year in research literature.
The name is a contraction of parallax second, and the definition follows directly from how stellar distances are actually measured. As the Earth orbits the Sun, nearby stars appear to shift slightly against the more distant background. Half of that annual shift is the star's parallax angle. A star with a parallax of one arcsecond lies at a distance of one parsec. The relationship is a simple reciprocal: distance in parsecs equals one divided by parallax in arcseconds, which is why the unit is convenient for observers. No conversion is needed between the measurement and the result.
Formally the parsec is defined as 648,000 divided by pi astronomical units, a value the International Astronomical Union fixed exactly in 2015. Herbert Hall Turner proposed the name in 1913.
No star is close enough to have a parallax of a full arcsecond. Proxima Centauri, the nearest, has a parallax of about 0.77 arcseconds and lies 1.3 parsecs away. The angles involved are minute, which is why parallax was not successfully measured until 1838 despite being sought since antiquity. The Gaia spacecraft has since measured parallaxes for over a billion stars with microarcsecond precision.
Multiples handle larger scales. The Milky Way is about 30 kiloparsecs across, and distances between galaxy clusters run to megaparsecs. The Hubble constant is conventionally quoted in kilometres per second per megaparsec.
The unit is widely known from a line in Star Wars in which the Millennium Falcon completes a route in under twelve parsecs, a distance where a time would be expected.
Cosmology works in megaparsecs and gives the unit a role in one of its central quarrels. The Hubble constant, the rate at which the universe expands, is quoted in kilometres per second per megaparsec: a galaxy one megaparsec away recedes at about seventy kilometres a second, one two megaparsecs away at twice that. Measurements from the cosmic microwave background give a value near 67, while those from supernovae and variable stars give about 73, and the gap between them has resisted a decade of effort to close it. Whatever resolves the disagreement, it will be argued over in this unit.
One parsec equals 3.26156 light years, 206,265 astronomical units, or 30.857 trillion kilometres.