Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Metre
open-in-new
<p>The metre is the <a href="/facts/SI_base_unit/M7PsTWg4">base unit</a> of <a href="/facts/Length/EwFyAqGH">length</a> in the <a href="/facts/International_System_of_Units/S7YLglNz">International System of Units</a> (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a <a href="/facts/Second/jtFxnJXx">second</a>, where the second is defined by a <a href="/facts/Caesium_standard/e8eu9bdv">hyperfine transition frequency of caesium</a>. </p><p>The metre was originally defined in 1791 by the <a href="/facts/National_Constituent_Assembly_(France)/obrvdx1R">French National Assembly</a> as one ten-millionth of the distance from the <a href="/facts/Equator/CjaxbVaN">equator</a> to the <a href="/facts/North_Pole/gjfItQJH">North Pole</a> along a <a href="/facts/Great_circle/x4fq6eNm">great circle</a>, so the <a href="/facts/Earth%27s_circumference/1b614Pg1">Earth's polar circumference</a> is approximately 40000 km. </p><p>In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of <a href="/facts/Krypton-86/8hRKYepJ">krypton-86</a>. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of <a href="/facts/Proper_length/vQyYYpsE">proper length</a>. From 1983 until 2019, the metre was formally defined as the length of the path travelled by <a href="/facts/Light/jeofpMn4">light</a> in vacuum in 1/299792458 of a <a href="/facts/Second/jtFxnJXx">second</a>. After the <a href="/facts/2019_revision_of_the_SI/g0h07jCX">2019 revision of the SI</a>, this definition was rephrased to include the definition of a second in terms of the caesium frequency Δ<i>ν</i>Cs. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures 40007.863 km, a change of about 200 <a href="/facts/Parts_per_million/PozRuoLM">parts per million</a> from the original value of exactly 40000 km, which also includes improvements in the accuracy of measuring the circumference. </p>