Question

How did peoples measured light's speed?

Answer 1

The speed of light in a vacuum stands at “exactly 299,792,458 metres per second“. The reason today we can put an exact figure on it is because the speed of light in a vacuum is a universal constant that has been measured with lasers; and when an experiment involves lasers, it’s hard to argue with the results. As to why it comes out somewhat conspicuously as a whole number, this is no coincidence- the length of metre is defined using this constant: “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.”