The Metre Is Now Defined by the Speed of Light
A metre is the base unit of length. Its definition has changed three times, and it is now fixed by a constant of nature.
A metre is the base unit of length in the metric system. Its definition has moved three times: originally one ten-millionth of the distance from the equator to the North Pole, then the length of a platinum-iridium bar kept near Paris, and since 1983 the distance light travels in a vacuum in 1/299,792,458 of a second. The definition now depends on nothing physical that could be lost or damaged. The awkward fraction exists so the new metre matched the old bar exactly.
- The base unit of length. *Two metres long.*
- Poetic rhythm. *Iambic metre.*
- Meter — the American spelling. *A meter.*
- Metric system — the system it anchors. *The metric system.*
- Greek — *metron*, a measure. *The source.*
Word forms
| Form | Word | Example |
|---|---|---|
| Noun (UK) | metre | One metre. |
| Noun (US) | meter | One meter. |
| Plural | metres | Ten metres. |
| Symbol | m | 5 m. |
| Greek | metron | Measure. |
- The pool is fifty metres long.Length.
- It is defined by the speed of light.Definition.
- The original was a fraction of the meridian.History.
Three definitions of the metre
- 1793 — one ten-millionth of the equator-to-pole distance
- 1889 — the length of a platinum-iridium prototype bar
- 1960 — a number of wavelengths of krypton-86 radiation
- 1983 — the distance light travels in 1/299,792,458 of a second
- Each step made the unit more reproducible anywhere on earth
French *mètre*, from Greek *metron*, a measure. Adopted as the base unit of the metric system in 1793.
Why is the speed of light such an odd number?
Because the metre was defined to match the existing standard. Fixing light's speed at 299,792,458 metres per second kept the old length unchanged.
Was the metre based on the earth?
Originally yes, as one ten-millionth of the distance from the equator to the pole. Later definitions abandoned that basis.