Unit of the month: METRE
A metre is the distance travelled by light in 1/299792458 of a second.
The reason we use the distance travelled by light in a certain amount of time is because light is the fastest thing in the universe (that we know of) and it always travels at exactly the same speed in a vacuum. This means that if you measure how far light has travelled in a vacuum in 1/299792458 of a second in France, Canada, Brazil or India, you will always get exactly the same answer no matter where you are!
On 20 May next year the official definition of the metre will change to:
The metre is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency ∆νCs.
So, what’s the difference? Actually, there’s no big change coming for the metre. Although the word order has been rephrased, the physical concepts remain the same.