"You've never heard of the Millennium Falcon? ... It's the ship that made the Kessel run in less than 12 parsecs!" Han Solo's description of the Millennium Falcon in Star Wars is impressive, but something's not quite right.
Do you know why? The unit he uses to illustrate the prowess of the Falcon - a parsec - isn't actually a measure of time, but length!

It probably won't surprise anyone Han Solo isn't very precise when it comes to the physics of his ship, but in fact he isn't too far from the truth. This is because we use time to define length. What does this mean?
Well, in the case of Han Solo, one parsec is about 3.26 light-years, and a light-year is the distance light travels in one year.

Back down on Earth, we have the same method for defining length. In the International System of Units (SI), the base unit of length is the metre, and it can be understood as: 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 2019 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 vacuumc to be 299 792 458 when expressed in the unit m s⁻¹, where the second is defined in terms of the caesium frequency ∆ν.

We'll be returning to the definition of the second on 20 March, so join us again then to find out more.

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.