How long is one meter?

I remember in a high school science class, my teacher lectured about SI units. She mentioned that in Paris there was a metal rod that was kept under very specific conditions, constant temperature, atmospheric pressure, etc. Why? Because this metal rod was the length of exactly one meter. This metal rod was the standard from which all other meter sticks were to be measured. I carried this image throughout my life and applied it to things non-scientific. Aux affaires du coeur, this attitude becomes difficult. There is no perfect meter stick. And keeping past meter sticks under the perfect conditions of your selective memory jeapordize your acceptance of the measurments you get from that meter stick you bought at the local HomeDepot. What my teacher failed to tell me was that since the turn of the 19th century, with the inception of the "meter", the definition of exactly what its length is has evolved. As our scientific knowledge has matured, so has our ability to more precisely define the exact length of a meter. So has the exact length of the meter changed over time? No, but we are better and more accurate when it comes to knowing what it is. Today a meter is defined as "the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second".