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In order to measure a length, we need a standard, and that standard is taken to be the meter. After the French Revolution In 1792, France established the meter as one ten-millionth of the distance from the equator to the north pole. This was a noble choice, but not a practical one, so the agreed upon distance was marked on a platinum-iridium bar, which is today still kept near Paris. More accurate requirements replaced this standard (in 1960) with another based on the emitted light from an atom, this time from krypton-86. That standard was later (in 1983) replaced by the distance light traveled in certain time. This, of course, causes a dilemma, because one standard (time) is being used to define another. This is resolved by defining the speed of light to be exactly 299,792,458 meters per second - so the speed of light is not a measured quantity, but a legislated quantity. ( Of course, our ability to measure the speed of light with extreme accuracy makes this possible.) The meter can thus be defined as the length of a light path (in vacuum) during a time interval of 1/299,792,458ths of a second.
Lengths of some familiar physical things range from 10-15 m (the radius of a proton - other particles are even smaller) to 1022 m (the distance to the Andromeda galaxy - a relatively nearby neighbor galaxy).