Kilometre

The kilometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: km) or kilometer (American spelling) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for 1000). It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the United Kingdom where the statute mile is the official unit used.

k (pronounced kay) is occasionally used in some English-speaking countries as an alternative for the word kilometre in everyday colloquialised writing and speech.^[1][2][3] A slang term for the kilometre in the U.S. military is klick.^[4]

Pronunciation

There are two common pronunciations for the word:

• /ˈkɪlɵmiːtər/ KIL-o-mee-tər and
• /kɨˈlɒmɨtər/ ki-LOM-i-tər

The former pronunciation follows the general pattern in English whereby metric units are pronounced with the stress on the first syllable, and the pronunciation of the actual base unit does not change irrespective of the prefix. It is generally preferred by the British Broadcasting Corporation (BBC) and the Australian Broadcasting Corporation (ABC).

Many scientists and other users, particularly in countries where the metric system is not widely used, use the pronunciation with stress on the second syllable.^[5][6] The latter pronunciation follows the stress pattern used for the names of measuring instruments (such as micrometer, barometer, thermometer, tachometer and speedometer). The problem with this reasoning, however, ^{[citation needed]} is that the word meter in those usages refers to a measuring device, not a unit of length. The contrast is even more obvious in countries using the English rather than American spelling of the word metre.

When Australia introduced the metric system in 1975, the first pronunciation was declared official by the government's Metric Conversion Board. However, the Australian Prime Minister at the time, Gough Whitlam, insisted that the second pronunciation was the correct one because of the Greek origins of the two parts of the word.^[7]

Equivalence to other units of length

1 kilometre	≡	1000	metres
	≈	3281	feet
	≈	1094	yards
	≈	0.621	miles
	≈	0.540	nautical miles
	≈	6.68×10−9	astronomical units[8]
	≈	1.06×10−13	light-years[9]
	≈	3.24×10−14	parsecs

History

Although the metre was formally defined in 1799, the myriametre (10,000 metres) was preferred to the "kilometre" for everyday use. The term "myriametré" appeared a number of times in the text of Develey's book Physique d'Emile: ou, Principes de la science de la nature,^[10] (published in 1802), while the term kilometre only appeared in an appendix. French maps published in 1835 had scales showing myriametres and "lieues de Poste" (Postal leagues of about 4288 metres)^[11]

The Dutch on the other hand adopted the kilometre in 1817 but gave it the local name of the mijl.^[12] It was only in 1867 that the term "kilometer" became the only official unit of measure in the Netherlands to represent 1000 metres.^[13]

Two German textbooks dated 1842^[14][15] and 1848^[16] respectively give a snapshot of the use of the kilometre across Europe - the kilometre was in use in the Netherlands and in Italy and the myriametre was in use in France.

In 1935, the International Committee for Weights and Measures (CIPM) officially abolished the prefix "myria-" and with it the "myriametre", leaving the kilometre as the recognised unit of length for measurements of that magnitude.^[17]

Visualisation

The kilometre may be visualised in terms of prominent landmarks.

Niagara Falls

Niagara Falls from Prospect Point. The extremity of the Horseshoe Falls in the distance is about one kilometre away The distance between the American extremity of the Niagara Falls and the Canadian extremity is 1039 m,[18] or slightly more than a kilometre. Although the length of the rim of the Canadian extremity (Horseshoe Falls) is quoted as being 790 m and the rim of the American extremity American Falls as being 320 m which add up to more than 1110 m, the rim of the Horseshoe falls is far from straight, so the direct distance between the end-points is 304 m. 43°05′08″N 79°04′08″W / 43.08557°N 79.06885°W / 43.08557; -79.06885 NE extremity of American Falls (Prospect Point) 43°04′44″N 79°04′41″W / 43.07901°N 79.07798°W / 43.07901; -79.07798 SW extremity of the Horseshoe Falls

The Mall, London

The Mall from Buckingham Palace on the occasion of the Queen's golden jubilee. The Victoria Memorial is in the foreground and the Admiralty Arch in the distance is about one kilometre away. The Mall, which leads up to Buckingham Palace, is one of London's main tourist attractions. Immediately in front of the palace is the Victoria Memorial, erected in memory of Queen Victoria (foreground of the picture to the right). At the opposite end is the Admiralty Arch which links The Mall to Trafalgar Square. The distance from the entrance to the Admiralty Arch to the centre of the Victoria Memorial is 987 m.[18] 51°30′07″N 0°08′26″W / 51.50185°N 0.14064°W / 51.50185; -0.14064 Centre of the Victoria Memorial 51°30′24″N 0°07′44″W / 51.50671°N 0.12876°W / 51.50671; -0.12876 Entrance to the Admiralty Arch

Kowloon – Hong Kong Crossing

Hong Kong – Tsim Sha Tsui Pier. The ferry terminal on the Kowloon Peninsular, visible in the centre of the picture, is about a kilometre from the Tsim Sha Tsui Ferry Pier. The Victoria Harbour separates two major areas of Hong Kong – Kowloon and Hong Kong Island. Until 1970 the only way to cross the harbour was the Star Ferry, which had terminals on both sides. The distance that the ferry travels, as measured from passenger entrance to passenger entrance is 1007 m.[18] 22°17′13″N 114°09′41″E / 22.28707°N 114.16133°E / 22.28707; 114.16133 Hong Kong (Central Star Ferry Pier) 22°17′37″N 114°10′05″E / 22.29371°N 114.16803°E / 22.29371; 114.16803 Kowloon (Tsim Sha Tsui Star Ferry Pier)

Central Park, Manhattan, New York City

Central Park in New York City is 849 metres wide. It extends (roughly east and west) from Fifth Avenue to Eighth Avenue (also known as Central Park West). The distance between Eighth Avenue and Madison Avenue (one block east of Fifth Avenue) is 1011 metres. The width of the park is visible from many nearby skyscrapers and is walked daily by many people. It is approximately 4 kilometres in length (from north to south).

The width of Central Park is visible from many buildings in Manhattan, and the park is walked regularly by many residents and visitors.

Suspension bridges

The Tagus Bridge in Lisbon, Portugal has a central span of 1.013 km A number of suspension bridges have a central span of a kilometre or more. The George Washington Bridge in New York (central span 1067 m) was the first bridge in the world to have a span of more than a kilometre and between 1931 and 1937, had the longest span of any bridge in the world. Other bridges that have a central span of about one kilometre include: 1030 m Third Kurushima-Kaikyo Bridge in Japan 1020 m Second Kurushima-Kaikyo Bridge in Japan 1013 m Ponte 25 de Abril (Tagus Bridge) in Lisbon, Portugal 1006 m Forth Road Bridge in Scotland  990 m Kita Bisan-Seto Bridge (Great Seto Bridge) in Japan  988 m Severn Bridge, England, leading up to the Welsh border.

International usage

Chinese expressway distances road sign in eastern Beijing. Although the primary text is in Chinese, the distances use internationally recognised characters.

The United Kingdom and the United States are the only two developed countries which continue to use miles on road signs.^[19]

United Kingdom

In the United Kingdom, road signs show distances in miles^[20][21] and location marker posts that are used for reference purposes by road engineers and emergency services show distance references in unspecified units which are kilometre-based.^[22] The advent of the mobile phone has been instrumental in the British Department for Transport authorising the use of driver location signs to convey the distance reference information of location marker posts to road users should they need to contact the emergency services.

United States

In the US, the National Highway System Designation Act of 1995 prohibits the use of federal-aid highway funds to convert existing signs or purchase new signs with metric units.^[23] The Executive Director of the US Federal Highway Administration, Jeffrey Paniati, wrote in a 2008 memo: "Section 205(c)(2) of the National Highway System (NHS) Designation Act of 1995 prohibited us from requiring any State DOT [Department of Transport] to use the metric system during project development activities. Although the State DOT's had the option of using metric measurements or dual units (metrics/inch-pounds), all of them abandoned metric measurements and reverted to sole use of inch-pound values."^[24] The Manual on Uniform Traffic Control Devices since 2000 is published in both metric and American Customary Units. (See also Metrication in the United States.)

Covering one kilometre

The time taken to cover a kilometre is dependent on one's speed. Walking on flat ground, a hiker can expect to cover one kilometre in 12 minutes^[25] while a car travelling at a speed of 120 kilometres per hour (75 mph)—the speed limit on numerous European expressways—will cover a kilometre in 30 seconds.

Some sporting disciplines feature 1000 m (one-kilometre) races in major events (such as the Olympic Games), but in other disciplines, even though world records are catalogued, the one kilometre event remains a minority event. The world records for various sporting disciplines are:

See also

• Conversion of units, for comparison with other units of length
• Cubic metre
• Orders of magnitude (length)
• Metric prefix
• Square kilometre

Notes and references

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