Originally Posted by Xaccers

Galileo tried and failed using two people with lanterns a mile apart
Roemer used Jupiters moons, but didn't have accurate distances so got it as 200,000Km/s
In 1958 Froome used a microwave interferometer and a Kerr cell shutter to measure the speed of light to be 299,792.5 km/s.
These days we define a meter as 1/299,792,458 the distance light travels in one second.
In A level physics, you can get away with using 3x10^8m/s