Definition of standard meter length has changed over centuries

You might not realize it, but certain fundamental units are the basis for all of our everyday measurements as well as those of the most precise scientific experiments. Physicists recognize seven fundamental units: mass, length, time, electric current, amount of matter, temperature and luminous intensity. These, either singly or in combination, describe everything physical that we can measure.
No one knows exactly why the fundamental units are fundamental or even why they are constant. There are