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The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; [1] SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, [2] is a unit of length in the International System of Units (SI) equalling 1 × 10 −6 metre (SI standard prefix "micro-" = 10 −6); that is, one millionth of a metre (or one thousandth of a ...
An early definition of the metre was one ten-millionth of the Earth quadrant, the distance from the North Pole to the Equator, measured along a meridian through Paris.. The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543.
One millionth is equal to 0.000 001, or 1 x 10 −6 in scientific notation. It is the reciprocal of a million , and can be also written as 1 ⁄ 1,000,000 . [ 1 ] Units using this fraction can be indicated using the prefix "micro-" from Greek , meaning "small". [ 2 ]
The prefixes are never combined, so for example a millionth of a metre is a micrometre, not a millimillimetre. Multiples of the kilogram are named as if the gram were the base unit, so a millionth of a kilogram is a milligram, not a microkilogram. [5]: 122 [11]: 14 The BIPM specifies 24 prefixes for the International System of Units (SI):
31 meters – wavelength of the broadcast radio shortwave band at 9.7 MHz; 32 meters – length of one arcsecond of latitude on the surface of the Earth; 33.3 meters – height of the De Noord, the tallest windmill in the world; 34 meters – height of the Split Point Lighthouse in Aireys Inlet, Victoria, Australia
In 1791 the commission originally defined the metre based on the size of the earth, equal to one ten-millionth of the distance from the equator to the North Pole. In the SI, the standard metre is now defined as exactly 1 ⁄ 299 792 458 of the distance that light travels in a second .
The picometre is one thousand femtometres, one thousandth of a nanometre ( 1 / 1000 nm), one millionth of a micrometre (also known as a micron), one billionth of a millimetre, and one trillionth of a metre. [2] The symbol μμ was once used for it. [3] It is also one hundredth of an ångström, an internationally known (but non-SI) unit ...