How timezones and navigation drove the first measurement of the speed of light

Astronomical observations of the Jupiter system — Jupiter and its moons — have had a rich history in navigation and astronomy. The first known observations of the Jupiter system were conducted by Galileo Galilei in 1610 who discovered the planets 4 largest moons: Io, Europa, Ganymede and Callisto in the same year. Galileo noted that the position of the moons were easy to predict, and their periods regular enough that they could be useful as a ‘celestial clock’. Jupiter and its moons were particularly useful in navigation for determining global position, a task that was notoriously difficult in Galileo’s time. Global position is specified through latitude and longitude. Renaissance sailors had already developed solutions for finding latitude through the use of astrolabes and sextants, but determining longitude remained a challenging problem. Determining longitudinal position was a pressing issue at the time since the discovery of Jupiter’s moons coincided with the age of discovery, a period of time when many European nations partook in extensive naval exploration. This was such a pressing issue that <a href="https://en.wikipedia.org/wiki/Longitude_rewards" rel="noopener ugc nofollow" target="_blank">the Spanish, Portuguese and English crowns would offer longitudinal prizes</a> for centuries to anyone capable of finding a suitable solution. <a href="https://medium.com/@domchang/jupiters-moons-and-the-speed-of-light-e86ee8b96d">Learn More</a>