Speed of light Historical speeds In a vacuum Commentary

Ole Rømer electromagnetism

Speed of light

Galileo had suggested using eclipses of the moons of Jupiter, as a celestial clock to determine longitude. Ole Rømer noted the times between eclipses of Io varied by the time of year, increasing as our distance to Jupiter increased and decreasing as our distance decreased. Realizing this meant that the speed of light is finite, Rømer calculated it using accumulated difference between actual and predicted eclipse times.

Historical speeds

Rømer’s result in 1676 was 26.6% slower than the true speed of light. James Bradley’s result in 1729 was 0.4% faster. Hippolyte Fizeau’s result in 1849 was 5.07% faster. Léon Foucault’s result in 1862 was 0.6% slower. Rosa and Dorsey’s result in 1907 was less than 0.03% slower.

In a vacuum

It’s somewhat crude today to say outer space is a perfect vacuum. It’s a sea of all kinds of things. And the speed of light in an atmosphere depends on altitude, temperature, and whether the light’s continuous or pulsed. Do absolutes exist in real life? We can never judge nor be judged in a vacuum. Take it with a grain of salt.