
As early as the 17th century, Galileo tried to measure the speed of light using lanterns, but without success. In 1676, Danish astronomer Ole Rømer, observing the moons of Jupiter, made the first estimate. Later, in the 19th century, James Clerk Maxwell created the theory of electromagnetism, which implied that light is an electromagnetic wave, and calculated its speed. The Michelson-Morley experiment showed that the speed of light does not depend on the motion of the source or observer.
Light propagates like ripples on water from a thrown stone, but in three dimensions and at colossal speed. This speed is embedded in the very fabric of space and time, so that the faster you move, the slower time passes for you — this ensures the constancy of the speed of light.