The Speed of Light

So today I thought I would talk a little about the history speed of light which I read about in Simon Singh's Big Bang. In the late 1600s one of the biggest questions for scientists was whether light had a finite or infinite speed. Although the solution is usually attributed to Ole Rømer, Jean-Dominique Cassini first proposed the idea that the speed of light was finite. You may be wondering how we could have possibly proven whether such a statement is correct. Well Cassini and Rømer did what many scientists of their age did--they looked to the sky. They studied the movements of Jupiter's moon Io which appeared (from Earth) to have an irregular orbit; Io's half moon was often several minutes before or behind its predicted appearance. What Rømer realized was that this irregularity could be explained if only the speed of light were finite. Since the Earth orbits the Sun 12x faster that Jupiter, Jupiter hardly moves while the Earth moves significantly. This means that sometimes Jupiter and Earth are at opposite sides of Earth's orbit and thus much further apart. If the speed of light had a finite value, then the significant variations in distance between Io and Earth would account for the half moon being early or late because the light would take more time to travel the extra distance when the two planets are at their furthest points. So Rømer spent three years recording the observed timings of Io and the relative positions of Earth and Jupiter. He then estimated that the speed of light had a finite value of 190,000 km/s (it is actually 300,000 km/s). Although somewhat inaccurate, this value was close enough that he accurately predicted that an eclipse of Io on November 9 1670 would be ten minutes late, thus proving that the speed of light was indeed finite.

The Paradoxicality