The kilometre (American spelling: kilometer), symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in 1⁄ 299,792.458 of a second. It is the conventionally used measurement unit for expressing distances between geographical places in most of the world; notable exceptions are the United States and the United Kingdom where the statute mile is used.
There are two pronunciations for the word:
The former pronunciation follows the general pattern in English whereby metric units of measurement are pronounced with the stress on the first syllable, and preserves the pronunciation of metre. It is generally preferred by the BBC, while most scientists use the pronunciation with stress on the second syllable. The latter pronunciation, which follows the stress pattern used for the names of measuring instruments (such as micrometer, barometer, thermometer, tachometer and speedometer), is in common usage as well. Kingsley Amis has suggested that this pronunciation (and the American spelling) be reserved for the thousand-measurer, the wall which Herodotus says Xerxes built around a thousand troops so he could count his army.
When Australia introduced the metric system, 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.
|1 kilometre||= 1000 metres|
|≈ 0.621 statute miles|
|≈ 1094 yards|
|≈ 3,281 feet|
|≈ 0.540 nautical miles|
|≈ 6.68 × 10−9 astronomical units|
|≈ 1.057 × 10−13 light-years|
|≈ 3.24 × 10−14 parsecs|
Since the kilometre is a comparatively large unit of measure, it is best visualised in terms of prominent landmarks.
The distance between the American extremity of the Niagara Falls and the Canadian extremity is 1039 m, or fractionally over 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.
NE extremity of American Falls
SW extremity of the Horseshoe Falls
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. 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. The picture on the right is taken from about halfway down The Mall.
Centre of the Victoria Memorial
Entrance to the Admiralty Arch
Hong Kong Harbour boasts one of the busiest waterways in the world. Until 1970 the only way to cross the harbour was the Star Ferry which had terminals on the Kowloon peninsular and the island of Hong Kong itself. The distance that the ferry travels, as measured from passenger entrance to passenger entrance is 1007 m.
Hong Kong (Central Star Ferry Pier)
Kowloon (Tsim Sha Tsui Star Ferry Pier)
The United Kingdom and the United States are the only two developed countries which continue to use miles on road signs.
Although road signs in the United Kingdom show distances in miles, location marker posts that are used for reference purposes by road engineers and emergency services are kilometre-based. The advent of the mobile phone has been instrumental in the English Department for Transport authorising the use of driver location signs to convey the [kilometre-based] information of location marker posts to road users should they need to contact the emergency services.
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. However, 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.)