# How did they calculate the speed of light?

Tags: light, speed
 P: 6 Just wondering as to the details of the experiment...
 Sci Advisor HW Helper P: 2,535 Well, a simple method is to take two clocks, synchonize them, seperate them, and then send pulses between them. If you record the distance and times for each event on each clock, then you can use d=r*t. Of course, that isn't really practical on laboratory scales, but it can be done using astronmical bodies where the distances are suitably large. I believe that one experiment involved recording the precise times that a moon of Jupiter was covered and uncovered by the planet, and extrapolating the speed of light from the change in distance and the knowledge of orbital motion.
 P: 12 The modern method for determining the speed of light was developed by K. M. Evenson et al. It relies on separate mesurements of wavelength and frequency emitted by a stabilized laser. They achieved the value c=2 999 792 458 m/s, which has uncertainty of only 1 m/s. Theoretically Maxwell predicted that self-supporting electromagnetic waves must propagate at speed $$c=\frac{1}{\sqrt{\mu_0\epsilon_0}}$$ for more info, check out: http://nvl.nist.gov/pub/nistpubs/sp958-lide/191-193.pdf
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PF Gold
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