Mile

A mile is a unit of length, usually used to measure distance, in a number of different systems, including Imperial units, United States customary units and Norwegian/Swedish mil. Its size can vary from system to system, but in each is between one and fifteen kilometers. In contemporary English contexts, mile most commonly refers to the international mile of 5,280 feet, 1,760 yards, or exactly 1,609.344 meters. However it can also refer to either of the following for specific uses:

• the U.S. survey mile (also known as U.S. statute mile) of 5,280 survey feet which is slightly longer at approximately 1,609.347219 meters (1 international mile is exactly 0.999998 survey mile).^[1][2]

• the international nautical mile of 1,852 meters (about 6,076 feet).^[3]

There have been several abbreviations for mile (with and without trailing period): mi, ml, m, M. In the United States, the National Institute of Standards and Technology now uses and recommends mi^[4] but in everyday usage (at least in the United States and in the United Kingdom) usages such as miles per hour and miles per gallon are almost always abbreviated as mph or mpg (rather than mi/h or mi/gal).

The formula "multiply by 8 and divide by 5" to convert international miles to kilometers gives a conversion of 1.6, accurate to 0.6%, which is a useful approximation.

Historical definitions

The unit of distance mille passus (literally "a thousand paces" in Latin, with one pace being equal to two steps) was first used by the Romans and denoted a distance of 1,000 paces or 5,000 Roman feet, and corresponded to about 1,480 meters, or 1,618 modern yards. This unit is now known as the Roman mile.^[5]

The current definition of a mile as 5,280 feet (as opposed to 5,000) dates to the 13th century, and was confirmed by statute in the reign of Queen Elizabeth I;^[6] However, various English-speaking countries maintained independent length standards for the yard, which differed by small but measurable amounts, and thus led to miles of slightly different lengths. This was resolved in 1959 with the definition of the current international mile by Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States.^[7]

Types of mile

In modern usage, various distances are referred to as miles.

International and statute miles

The international mile (and before 1959, the statute mile) is the distance typically meant when the word mile is used without other qualifying words (e.g. nautical mile, see below). The international and statute miles are both equal to 5,280 feet, but the international mile is defined in terms of the international foot (0.3048 m), while the statute miles of the various English-speaking countries were based on the national foot of each country. The (mostly obsolete) U.S. statute mile is based on the U.S. survey foot (which is exactly 1200/3937 m) and differs from the international mile by about 3 mm. ^[8]

The name statute mile originates from a statute of the Parliament of England in 1592 during the reign of Elizabeth I. This defined the statute mile as 5,280 ft or 1,760 yards; or 63,360 inches. Both statute and international miles are divided into eight furlongs (the length generally that a furrow was ploughed before the horses were turned, furlong = furrow-long). In turn a furlong is ten chains (a surveyor's chain, used as such until laser range finders took over); a chain is 22 yards and a yard is three feet, making up 5,280 ft.

Other miles in Britain and Ireland

Before the statute of the English parliament, there was confusion on the length of the "mile". The Irish mile was 6,721 feet and the Scottish mile was 5,951 feet.^[9] Perhaps the earliest tables of English linear measures, Arnold's Customs of London (c. 1500) indicates a mile consisted of 8 furlongs, each of 625 feet, for a total of 5,000 feet.^[10] For other "miles" see the list below.

Nautical miles

On the utility of the nautical mile
Each circle shown is a great circle – the analog of a line in spherical trigonometry – and hence the shortest path connecting two points on the globular surface. Meridians are great circles that pass through the poles.

Main article: Nautical mile

The nautical mile was originally defined as one minute of arc along a meridian of the Earth.^[11] It is a convenient reference since it is fairly constant at all latitudes, in contrast with degrees of longitude which vary from from 1 NM at the equator to zero at the poles.

Navigators use dividers to step off the distance between two points on the navigational chart, then place the open dividers against the minutes-of-latitude scale at the edge of the chart, and read off the distance in nautical miles.^[12] Since it is now known that the Earth is not perfectly spherical but an oblate spheroid, the length derived from this method varies slightly from the equator to the poles. For instance, using the WGS84 Ellipsoid, the commonly accepted Earth model for many purposes today, one minute of latitude at the WGS84 equator is 6,087 feet and at the poles is 6,067 feet. On average it is about 6,076 feet (about 1852 meters or 1.15 statute miles).

In the United States of America, the nautical mile was defined in the nineteenth century as 6,080.2 feet (1,853.249 m), whereas in the United Kingdom the Admiralty nautical mile was defined as 6,080 feet (1,853.184 m) and was approximately one minute of latitude in the latitudes of the south of the UK. Other nations had different definitions of the nautical mile, but it is now internationally defined to be exactly 1,852 meters.

The nautical mile per hour is known as the knot. Nautical miles and knots are almost universally used for aeronautical and maritime navigation because of their relationship with degrees and minutes of latitude and the convenience of using the latitude scale on a map for distance measuring.

Other miles

• The Danish mil (traditional) was 24,000 Danish feet or 7.5325 kilometers. Sometimes it was interpreted as exactly 7.5 kilometers. It is the same as the north German Meile (below).^[13]
• The data mile is used in radar-related subjects and is equal to 6,000 feet (1.8288 kilometers).^[14]
• The Meile was a traditional unit in German speaking countries, much longer than a western European mile. It was 24,000 German feet; the SI equivalent was 7586 meters in Austria or 7532.5 meters in northern Germany. There was a version known as the geographische Meile which was 4 Admiralty nautical miles, 7,412.7 meters, or 1/15 degree.^[15]
• The term metric mile is used in sports such as athletics (track and field) and speed skating to denote a distance of 1.5 kilometers. In United States high school competition the term is sometimes used for a race of 1.6 kilometers.^[16]
• In Norway and Sweden a mil is a unit of length which is equal to 10 kilometers and commonly used in everyday language. However in more formal situations, like on road signs and when there is risk of confusion with English miles, kilometers are used instead. The traditional Swedish mil spanned the range from 6000-14,485 meters, depending on province. It was however standardized in 1649 to 36,000 Swedish feet, or 10.687 kilometers^[17]. The Norwegian mil was 11.298 kilometers. When the metric system was introduced in the Norwegian-Swedish union in 1889, one standardized the mil to exactly 10 kilometers ^[18]

• The radar mile is a unit of time, equal to the time required for a radar pulse to travel a distance of two miles (one mile each way). Thus, the radar statute mile is 10.8 μs and the radar nautical mile is 12.4 μs.^[19]
• The Russian milya (русская миля) was a traditional Russian unit of distance, equal to 7 verst, or 7.468 km
• The hrvatska milja (Croatian mile) is 11,130 metres = 11.13 kilometres = 1/10 of equator's degree.^[20], first time used by Jesuit Stjepan Glavač on map from 1673.
• The banska milja (also called hrvatska milja) (mile of Croatian Ban, Croatian mile) was 7586 metres=7,586 kilometres, or 24,000 feet.^[21].

Idioms

Even in countries that have moved from the Imperial to the Metric system (for example, Australia and New Zealand), the mile is still used in a variety of idioms. These include:

• A country mile is used colloquially to denote a very long distance.
• "A miss is as good as a mile" (Failure by a narrow margin is no better than any other failure)
• "Give him an inch and he'll take a mile" (The person in question will become greedy if shown generosity)
• "Missed by a mile" (Missed by a wide margin)
• "Talk a mile a minute" (Speak at a rapid rate)
• "To go the extra mile" (To put in extra effort)
• "Miles away" (Lost in thought, or daydreaming)

See also

Notes

References

• Astin. V. and H. Arnold Karo. (1959). Refinement of values for the yard and the pound, Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59-5442, Filed, June 30, 1959, 8:45 a.m.)

• Barbrow, Louis E. and Lewis V. Judson (1976). Weights and Measures Standards of the United States: a brief history. National Institute of Standards and Technology.

• Butcher, Tina et al. ed. (2007). NIST Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. Appendix C, p. C-13.

• Klein, Herbert Arthur (1974, corrected 1988). The Science of Measurement: A Historical Survey. New York: Dover. (previously published by Simon & Schuster under the title The World of Measurements: Masterpieces, Mysteries and Muddles of Metrology)

• Maloney, Elbert S. (1978). Dutton's Navigation and Piloting. 13th Ed. Annapolis: Naval Institute Press.

• Rowlet, Russ (2005). How Many? A Dictionary of Units of Measurement. Faculty member's web page at University of North Carolina at Chapel Hill. Accessed 2007-11-10.

• Smith, William (1875). Dictionary of Greek and Roman Antiquities. London: John Murray. pp. 762. 

External links

• NIST General Tables of Units of Measurement