How Romer calculated Speed of Light
... and that's how Rømer calculated Speed of Light Rømer found differences in Jupiter's Eclipses of Io. The confusion here is, the total time of a eclipse will always be same regardless the earth's position around the sun so how did he get the time difference. Jupiter is so large that eclipses occur at regular intervals. Every time Io completes a revolution around Jupiter, the appearing time and disappearing (behind Jupiter) time can be observed by a telescope. Rømer predicted all of Jupiter's eclipses for the next year. But as the Earth was moving away from Jupiter, there was the time difference between predicted starting time of eclipse and actual starting time of Eclipse. The maximum difference was 22 minutes. Rømer concluded that Io 's Orbital Period cannot be changed. The only possibility is Speed of Light is not infinite. He noticed that when Earth moved away from Jupiter, the observed eclipses of Io were delayed by up to 22 minutes compared to when Earth w...