SI definition of the metre

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

The dimension of length is [L], its unit is the metre and its symbol m.

The original definition of the metre was one ten millionth of the distance from the Earth's north pole to its equator, determined along a meridian arc that ran from Dunkirk in the north to Barcelona in the south. Observations were begun in 1792 by J. B. J. Delambre, who worked from Paris northwards and P. F. A. Méchain who made measurements from Paris to Barcelona. They completed their task in seven years and the metre thus determined was modelled in pure platinum as a one-metre-long bar (Alder, 2004).

Linear astronomical distances and diameters

The sizes of and the distances between astronomical bodies is generally extremely large by everyday terrestrial standards. This has resulted in astronomers inventing units such as the light year, the astronomical unit and the parsec, which are, at first sight, better able to deal with very large distances. The SI unit of length, the metre, used in conjunction with common prefixes is normally only used for measurements within the Solar System.

Size of the Earth

Were the Earth a perfect sphere it would follow from the original definition of the metre that its diameter would be 4 × 107/π m.