Eli5 : Why do we define metre as length of path of light in vaccum during a time interval of 1/299,792,458 of a second ?

Question

Why not 1/300,000,000 or something different constant ( why only light ) ?

In: Physics

Answer 1

Because we defined the size of a metre before we knew what the speed of light is. Instead of changing the size of the metre, we used a more accurate way of measuring the same length.

We use the speed of light because it is constant, and we can measure time way more accurately than we can measure distance.