# How did they calculate the speed of light?

by crystal cascade
Tags: light, speed
 P: 6 Just wondering as to the details of the experiment...
 Sci Advisor HW Helper P: 2,538 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
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,886

## How did they calculate the speed of light?

Here's a very nice website that answers your question:
http://www.what-is-the-speed-of-light.com/

The first accurate measurement of the speed of light was done by timing when Jupiter's moons moved out from behind it.
 P: 6 wow thanks :) I knew one of the experiments had something to do with the moons of jupiter...the one my teacher wanted us to research was Mickelson's

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