In 1931, Albert Michelson devised a method of measuring the
speed of light, directly, by finding how long it took to
move a measured distance. 
The diagram below, showing the approximate arrangement of the
apparatus, is not to scale (but I assume you guessed that!) 



Light from the source passes through a narrow slit. 
It is reflected by face A of the octagonal (8 sided) metal prism. 
It then travels a distance of a few kilometres and
returns to be reflected by face B. 
When the prism is stationary, a stationary image of the slit is
observed. 
The prism is now rotated. 
If the prism rotates fast enough, when light returns to
the prism, face B is no longer in the right position to reflect it
into the observer’s eye. 
The image of the slit disappears.

However, if the speed of rotation is increased, at a certain
speed of rotation, the image of the slit reappears. 
This is because the time taken for light to go from face A to
face B was the same as the time taken by the prism to rotate 1/8^{th}
of a revolution. 
So, the calculation needed to find the speed of light is
remarkably simple: 
If the prism completes n rotations per second then 

Therefore, the time, t taken for the light to cover the
distance, s is 1/8^{th} of
this 

and so the speed of light c is given by 

In 1931, Michelson found c = 2.99774×10^{8}ms^{1} 
The modern value is c = 2.997925×10^{8}ms^{1} 

Although the calculation is trivially
simple don't forget the practical difficulties involved in this
experiment: for example, measuring the distance moved
by the light to a satisfactory level of precision... 