Question

The metre is defined as the distance travelled by light in $\frac{1}{299,792,458}$ second. Why didn't people choose some easier number such as $\frac{1}{300,000,000}$ second? Why not 1 second?

Answer 1

The speed of light in vacuum is 299,792,458 m/s.
Then time taken by light to cover a distance of 1 metre in vacuum = $\frac{1}{299,792,458}$ s
Hence, the metre is defined as the distance travelled by light in $\frac{1}{299,792,458}$ s.
As 300,000,000 m/s is an approximate speed of light in vacuum, it cannot be used to define the metre.

The distance travelled by light in one second is 299,792,458 m. This is a large quantity and cannot be used as a base unit. So, the metre is not defined in terms of second.