Summary
They used the distance from the Earth’s north pole to the south pole as the first idea.
Scientists have changed the exact definition many times to make it more precise.
Key Points
3 points1
The first metre was based on the Earth’s north‑to‑south distance.
2
Later measurements needed more exact tools, so the definition was updated.
3
Today the metre is defined using the speed of light.
New Words to Learn
4 wordsmetre
A unit of length, about the length of a large ruler.
definition
A precise explanation of what something means.
meridian
An imaginary line that goes from the top of the Earth to the bottom.
precise
Very exact and accurate.
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