Short Story: Galileo's Discovery Of The Moons Of Jupiter

From your car racing down the freeway, to the sound of the sirens you hear from the cop closing the gap, everything has a speed limit. But for thousands of years the most brilliant minds of history didn’t believe this. They thought that light had no limits. That it was instantaneous. Until around three and half centuries ago, that though was challenged. It essentially began with Galileo’s discovery of the moons of Jupiter, more specifically the discovery of Io. He noticed the moons were quite periodic. It didn’t take long for astronomers to realize this was a fairly accurate way to keep time. It was known to great accuracy that Io orbited Jupiter every 42.5 hours. So if we were to observe Io during this class period as I’m giving my speech at 11:30 am on Thursday, then you would see it at precisely the same position at 8:00 pm the following day.
What he realize was that it takes time for light to travel from Jupiter to the Earth. So when the Earth was on the opposite side of the sun the light had a greater distance to travel, leading to seeing Io moments later. Vice versa, with the other configuration, the shorter distance accounted for an earlier sighting of Io. What Roemer had just discovered was that light doesn’t travel instantaneously, light has a finite speed through space. This understanding of his lead to a measurement of the speed of light. We now know the speed of light, which is 2.99*10^10 cm/s. Conceptually one of my favorite ways to imagine how fast that really is an example I had read from Professor Brian Cox’s book The Wonders of The Universe a few years ago. The time it takes me to snap my fingers light has traveled around the Earth 7 times. This measurement also shed light on another fascinating idea, this can be used to measure distance and time. The light