---
title: "The last time — science and measurement"
description: "5 things in science and measurement that happened once and have not happened since, with the date and what actually took place."
url: https://atoneplace.net/the-last-time/science/
site: "At One Place"
---

# The last time: science and measurement

5 closed last occurrences in science and measurement, most recent first.

| Question | When | What | Could it change? |
| --- | --- | --- | --- |
| When were the SI base units last redefined? | 20 May 2019 | The kilogram, ampere, kelvin and mole were all redefined at once | No |
| When was the last leap second added? | 31 December 2016 | A second added to the end of 31 December | Yes |
| When was an element last added to the periodic table? | 28 November 2016 | Nihonium, moscovium, tennessine and oganesson were named, completing row seven | Yes |
| When did the number of planets last change? | 24 August 2006 | Pluto was reclassified as a dwarf planet, leaving eight | No |
| When was the metre last redefined? | 1983 | Defined as the distance light travels in 1/299,792,458 of a second | No |

- [When were the SI base units last redefined?](https://atoneplace.net/the-last-time/si-base-units/) — 20 May 2019
- [When was the last leap second added?](https://atoneplace.net/the-last-time/leap-second/) — 31 December 2016
- [When was an element last added to the periodic table?](https://atoneplace.net/the-last-time/new-element/) — 28 November 2016
- [When did the number of planets last change?](https://atoneplace.net/the-last-time/planet-count/) — 24 August 2006
- [When was the metre last redefined?](https://atoneplace.net/the-last-time/metre-redefined/) — 1983

## Related pages

- [Space](https://atoneplace.net/the-last-time/space/)
- [Countries and borders](https://atoneplace.net/the-last-time/nations/)
- [Technology that stopped](https://atoneplace.net/the-last-time/technology/)
- [The natural world](https://atoneplace.net/the-last-time/nature/)
- [Everyday things](https://atoneplace.net/the-last-time/everyday/)