# meter (m)

The meter (m) is the base unit of length in the SI system of units. 1 meter = 39.3701 inches. It is defined by the International Bureau of Weights and Measures as the distance traveled by light in a vacuum in 1/299,792,458 of a second.

A **kilometer** (1 km) = 1,000 m = 0.62 mile. Alternatively, 1 mile = 1.609 km.

A **micron** is a unit of length in the CGS system, equal
to one millionth of a meter. In SI units
it is replaced by the micrometer (μm).

One billionth (10^{-9}) of a meter.
It used, for example, in the measurement of intermolecular distances and wavelengths.
A sheet of paper is about 100,000 nanometers thick. The nanometer has superceded
the angstrom as the accepted unit
for such measurements.

## History of the meter

It was originally intended that the meter ('meter' in European
spelling) represent one ten millionth of the distance from the North Pole
to the equator on the meridian passing through Paris. But the surveyors
got their sums wrong and for 162 years (to 1960), the meter was defined
as an arbitrary distance between two marks on a bar of platinum kept in Paris – the so-called "international prototype metre". From
1960 to 1983, the meter was defined as the length equal to 1,650,763.73
times the wavelength of radiation corresponding to the transition between
the energy levels 2p_{10} and 5d_{5} of the krypton-86 atom.