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    One word: interferometery.

    Have a look at this link:

    http://www.explainthatstuff.com/howi...eterswork.html

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    It's defined as "the length of the path travelled by light in a vacuum in a time of 1 / 299792458 second".
    The great precision and reliability of atomic clocks (in measuring that time interval) has made this definition possible.
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    Interestingly, it was originally decided upon as being one ten-millionth of the distance between the equator and the North Pole. The definition has been consistently changed to improve the accuracy over the centuries, the latest definition being based on the speed of light.
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    (Original post by Actaeon)
    Interestingly, it was originally decided upon as being one ten-millionth of the distance between the equator and the North Pole. The definition has been consistently changed to improve the accuracy over the centuries, the latest definition being based on the speed of light.
    Through the middle of Paris... because the French had meridian envy of Greenwich

    There was a BBC series on all this a while back... but not recently enough to still be on iplayer
    http://www.bbc.co.uk/programmes/b02xbjmf

    might be a youtube
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    (Original post by Joinedup)
    Through the middle of Paris... because the French had meridian envy of Greenwich

    There was a BBC series on all this a while back... but not recently enough to still be on iplayer
    http://www.bbc.co.uk/programmes/b02xbjmf

    might be a youtube
    I think I watched that when it was first on... I do like Marcus du Sautoy's programs, he's done quite a few interesting ones. As well as having a cool name
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    (Original post by Joinedup)
    Through the middle of Paris... because the French had meridian envy of Greenwich

    There was a BBC series on all this a while back... but not recently enough to still be on iplayer
    http://www.bbc.co.uk/programmes/b02xbjmf

    might be a youtube
    Wasn't it through Dunkirk?
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    (Original post by Stonebridge)
    Wasn't it through Dunkirk?
    NIST says says that it's Paris :unsure: http://physics.nist.gov/cuu/Units/meter.html
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    (Original post by Joinedup)
    NIST says says that it's Paris :unsure: http://physics.nist.gov/cuu/Units/meter.html
    Thanks. It was both it seems.
    I knew I had Dunkirk locked away in my memory somewhere.
    The full story here
    http://www.french-metrology.com/en/h...-adventure.asp
 
 
 
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