The Velocity of Light, and Light in Moving Frames of Reference

In 1676, Olaf Romer, a Dane, at that time tutor to the royal princes at the court of Louis XIV in Paris, discovered the finite propagation velocity of light. Based on astronomical observations, he obtained a value which is of the correct order of magnitude, namely \(c=2.3\cdot 10^{8}\) m/s. He used light signals from one of the moons of Jupiter at the moment when it emerged from the shadow of the planet. The delay between successive signals was found to be 42.5 h, i.e. equal to the time required for one orbital passage of the Jupiter moon. He carried out the measurements when Jupiter was at its closest approach to the earth and when it was furthest away, that is at opposite points along the earth’s orbit around the sun, separated by the diameter of the orbit (the diameter of earth’s orbit is \(3\cdot 10^{11}\) m). At the most distant point on the orbit, the signal showed an additional delay of 1320 s. From this, he was able to calculate the above value of the velocity of light.