Today And For A Few Hundred Years,

we have known the speed of light,

image credit NASA/ Wikimedia Commons, light in a vacuum travels at 186,282 miles per second (299,792 kilometres per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second, but how did we find out about how fast it travelled? back in the day, scientists believed that light was infinitely fast; it travelled instantaneously. But 344 years ago, on November 21, 1676, a Danish astronomer named Ole Rømer disproved that. When he studied Io, one of Jupiter’s moons, he discovered that light did not travel instantaneously, and light had a finite speed, He was trying to figure out how long it takes Io to orbit Jupiter in hopes of using it as a cosmic clock. He watched Io disappear behind Jupiter and reappear on the other side. He did this over and over every 42 hours for years, to his surprise, the timing of the eclipses was not consistent. When Earth was closest to Jupiter, the eclipses happened 11 minutes early. Likewise, when the two planets were farthest away, the eclipses were 11 minutes behind schedule, Rømer figured out the pattern and made an accurate prediction for Io's eclipse on Nov. 9, 1676. Then on Nov. 21, he took his findings to the Royal Academy of Sciences and explained that a finite speed of light must be responsible, what an amazing piece of observation.