When was the metre last redefined?
When was the metre last redefined?
1983
Defined as the distance light travels in 1/299,792,458 of a second
The consequence is that the speed of light is no longer measured — it is fixed by definition at exactly 299,792,458 metres per second, and any improvement in measurement now refines the metre rather than the speed. Before 1983 the metre was defined by a krypton-86 emission wavelength, and before that by a metal bar. The bar version, from 1889, was itself an approximation of an eighteenth-century calculation of one ten-millionth of the distance from the pole to the equator, and it was slightly wrong.
- What
- Defined as the distance light travels in 1/299,792,458 of a second
- When
- 1983
- Category
- Science and measurement
- Could this change?
- Not in any foreseeable way. The event is closed.
Science and measurement
| Question | When |
|---|---|
| When were the SI base units last redefined? | 20 May 2019 |
| When was the last leap second added? | 31 December 2016 |
| When was an element last added to the periodic table? | 28 November 2016 |
| When did the number of planets last change? | 24 August 2006 |
This page gives a date and never a "years ago". These pages are built as static files, so a computed interval would freeze at build time and quietly become wrong — which on a reference site is worse than making the reader do the subtraction.
Common questions
- When was the metre last redefined?
- 1983 — Defined as the distance light travels in 1/299,792,458 of a second.
- What happened?
- The consequence is that the speed of light is no longer measured — it is fixed by definition at exactly 299,792,458 metres per second, and any improvement in measurement now refines the metre rather than the speed. Before 1983 the metre was defined by a krypton-86 emission wavelength, and before that by a metal bar. The bar version, from 1889, was itself an approximation of an eighteenth-century calculation of one ten-millionth of the distance from the pole to the equator, and it was slightly wrong.
Related pages
- When were the SI base units last redefined? 20 May 2019
- When was the last leap second added? 31 December 2016
- When was an element last added to the periodic table? 28 November 2016
- When did the number of planets last change? 24 August 2006
- When did the number of planets last change? Also 24 August 2006
- When did Concorde last fly? Also 24 October 2003, with the last flight of all on 26 November 2003
- When was the US Constitution last amended? Also 7 May 1992
- When was Halley's Comet last visible? Also 9 February 1986
- When did the prime meridian last move? Also 1984
- When did a human last walk on the Moon? Also 14 December 1972
- Everything, most recent first 36 last occurrences
Sources
- Guide for the Use of the International System of Units (SP 811) — NIST (Public domain (US Government))