How is the speed of light measured?
There are several ways of measuring the speed of light. It is now a constant.
The speed of light was first measured by measuring the time differences between observed and predicted eclipses of Jupiter's moons.
It was later calculated by by shining light at distant rotating mirrors.
When Maxwell formulated his equations the speed of light could be calculated directly from the equation:
The second has been well defined in terms of transitions of caesium atoms. The metre was not sufficiently well defined. To correct this the speed of light is now defined to be exactly 299 792 458 m/s. The metres is now defined to be the distance light travels in 1/299 792 458 seconds.