Summary
The metre has been re‑defined several times as measurement technology improved.
Today the metre is defined by the distance light travels in a fraction of a second.
These updates keep the metre accurate for science and everyday use.
Key Points
4 points1
1793: First metre defined using the Earth’s north‑to‑south line.
2
Later definitions used physical objects like metal bars.
3
Modern definition relies on the constant speed of light.
4
The current definition makes measurements extremely reliable.
New Words to Learn
5 wordsmetre
The basic unit for measuring length, similar to a big ruler.
definition
A clear, exact meaning of a term.
meridian
An imaginary line that connects the North and South Poles of the Earth.
fraction of a second
A very tiny part of one second, much shorter than a blink.
speed of light
How fast light travels, which is a constant and very fast.
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