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Kilometre

physical science Maturity 11-13 Next article →

The kilometre is a fundamental unit of length. It belongs to the International System of Units, known as the SI. In American English, it is spelled kilometer. This unit is essential for measuring distances on land. Most countries in the world use it to express how far apart geographical places are. However, the United States and the United Kingdom are notable exceptions. These nations prefer to use the statute mile instead.

Division of the meridien
Division of the meridien

To understand the kilometre, you must understand its name. The word uses the SI prefix "kilo-". In this system, "kilo-" means one thousand. Therefore, one kilometre is exactly equal to one thousand metres. The official symbol for this unit is "km". This symbol is always written in lower case. The International Bureau of Weights and Measures, or BIPM, has standardised this symbol.

There is a debate regarding how to pronounce the word. One common way follows a specific pattern used for other SI units. In this version, the stress is placed on the first syllable. This matches words like kilogram, kilojoule, or kilohertz. This pronunciation is preferred by organizations like the BBC, CBC, and ABC. The second way places the stress on the second syllable. This pattern is similar to measuring instruments like a thermometer or a barometer. This second way is considered irregular for an SI unit. In Australia, a dispute even occurred over this. The government's Metric Conversion Board declared the first way official in 1970. However, Prime Minister Gough Whitlam insisted the second way was correct due to Greek origins.

The history of this measurement began in France. On 8 May 1790, the French National Constituent Assembly issued a decree. They ordered the French Academy of Sciences to create a new system. By August 1793, the French National Convention made the metre the sole length measurement. This metre was based on a specific scientific calculation. It was one millionth of the distance from the orbital poles to the Equator. This was intended to be a truly international unit.

Division of the meridien
Division of the meridien

Early names for the kilometre were quite different. Its first name was "Millaire." For many years, people preferred a different unit called the myriametre. A myriametre is equal to ten thousand metres. This term appeared in the 1802 book Physique d'Emile. In 1835, French maps used both myriametres and "lieues de Poste." These postal leagues were about 4 kilometres long. Eventually, the myriametre was removed from use. In 1935, the International Committee for Weights and Measures officially abolished the prefix "myria-". This left the kilometre as the recognised unit for that scale.

Different European nations adopted the unit at different times. The Dutch began using the kilometre in 1817. They initially gave it the local name "mijl." It was not until 1867 that it became the only official unit in the Netherlands. Textbooks from 1842 and 1848 show how the unit spread. By that time, the kilometre was used in Italy and the Netherlands. Meanwhile, France was still using the myriametre. This shows how slowly measurement standards can change across borders.

We can compare the kilometre to many other units of length. One kilometre is approximately 0.621 miles. It is also about 0.540 nautical miles. When looking at much larger scales, the kilometre is quite small. It is much smaller than an astronomical unit. It is also much smaller than a light-year or a parsec. In certain circles, the word even has a slang version. Soldiers in the US, UK, and Canadian militaries often call a kilometre a "klick."

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