As a point of interest, there have been numerous measurements of the
speed of light. Einstein did rely on this constancy and I thought this
would be of interest as to all the measurements made over the last +300
years.
When was the speed of light first measured?;
http://zebu.uoregon.edu/~soper/Light/speedoflight.html
Light moves with a speed c = 3 x 108 m/s.
This speed was first measured 1n 1675 by Danish astronomer Olaus Roemer,
using observations of the moons of Jupiter.
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The Fizeau/Foucault measurement in 1868;
http://www.astro.virginia.edu/~eww6n/bios/Fizeau.html
French physicist who developed a toothed disk known as the Fizeau Wheel
with which he and Foucault measured the speed of light. In 1868, he
suggested using an interferometric method to measure stellar diameters
by placing a mask with two holes in front of a telescope's aperture.
He calculated that the fringes would vanish at a separation related to
the size of the Star. Fizeau's ideas were pursued unsuccessfully by
Stephan and, using a different concept, successfully by Michelson.
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First accurate measurement of light and includes Michelson;
http://faculty.millikin.edu/~jaskill.nsm.faculty.mu/c.html
The first accurate measurement for the speed of light was made over a
century later in 1849, by the French scientist Armande Fizeau
(1819-1896).
He used a rotating toothed wheel to chop the light from a lantern and
sent it over a distance of more than 5 miles to a mirror that reflected
the light back to the source. The speed of revolution of the wheel was
adjusted so that the reflected light could be seen through the spaces in
the wheel.