Rømer's Law

Rømer's Law explains how we determine the speed of light by observing eclipses of Jupiter's moon Io. In 1676, Ole Rømer noticed that the time between Io's eclipses varied depending on the Earth’s position relative to Jupiter. When Earth was moving away, the eclipses took longer; when moving closer, they happened sooner. By measuring these time differences, Rømer estimated that light takes about 22 minutes to cross the distance between the Sun and Earth, leading to the understanding that light has a finite speed. This discovery provided the first quantitative estimate of the speed of light.