Speed Of Light

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Designated c, the speed of light is about $3\times10^8$ m/s, or about a foot per nanosecond. That's a million times faster than the speed of sound.

In 1638 Galileo tried to estimate it by having an assistant uncover a lantern on a distant (1 mile) hilltop at a predetermined time, but it only takes light 5 micro-seconds, so he would have had to have been fast on the stopwatch.

In 1676 Ole Roemer used differences in the times of the eclipses of the Moons of Jupiter to estimate it, and James Bradley used stellar aberration in 1728.

The speed of light and measuring the difference in time taken to travel from different places is what makes GPS work.

The equations of Einstein's Special Theory of Relativity have a singularity at the speed of light indicating that no object can travel faster than the speed of light.

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