How did they originally discover the speed of light?

The speed of light in a vacuum stands at “exactly 299792458 metres per second“. read more

Best answer comes from How the Speed of Light was First Measured It wasn’t until Danish Astronomer, Ole Römer entered the fray that measurements of the speed of light got serious. In an experiment that made Galileo flashing lanterns on a hill look like a primary school science fair project, Römer determined that, lacking lasers and explosions, an experiment should always involve outer space. read more

"Einstein's work advanced the principle that the speed of light is constant in a vacuum and that strange thing occurs the nearer to its speed you reach. Including effects such as time dilation or the slowing of time the faster you travel. The speed of light seems to be the fastest a body with mass can travel. read more

It was the Danish astronomer, Olaus Roemer, who, in 1676, first successfully measured the speed of light. His method was based on observations of the eclipses of the moons of Jupiter (by Jupiter). read more

Rømer did try to measure the speed of light. In fact, he was the firs to prove that the speed is finite. But his measurement was with 213.000 Kilometre per second rather inaccurate. 1728 James Bradley measured the speed of light at 301.000 what could be seen as the first rather accurate measurement. read more