The man who measured the speed of light… by observing a moon of Jupiter.

Long before lasers or atomic clocks existed, it was already suspected that light was not instantaneous. Galileo Galilei, in the 17th century, tried to measure its speed using lanterns, but concluded that if it had a speed, it was too fast to detect with his methods. He was not wrong… he just lacked precision.

Decades later, in 1676, the Danish astronomer Ole Rømer took a decisive step. While studying the eclipses of Io, a moon of Jupiter, he noticed that they did not always occur at the same moment: when the Earth was moving away from Jupiter, Io seemed to take longer to disappear; when it was getting closer, the eclipse arrived sooner.

Rømer's brilliance was in asking the right question:

What if the delay was not from Io… but from us?

What if light simply needed more time to arrive, as the distance between the Earth and Jupiter increased?

With this revolutionary idea, he proposed that light had a finite speed and estimated it at 220,000 kilometers per second. Although this is not the exact value we accept today (299,792 km/s), it is not far off, considering the tools of the 17th century. It was the first time in history that someone managed to measure the speed of light using astronomical observations.