Google Doodle marks determination of speed of light 340 years ago

Google Doodle has celebrated the occasion of determination of speed of light 340 years ago with a Doodle on Wednesday. Danish astronomer Ole Roemer determined the speed of light - 299,792,458 metres per second - in 1676. For NASA Astronomy Cafe, part of the NASA Education and Public Outreach programme, Sten Odenwald explained that in 1676, Roemer was studying the eclipses of Jupiter's moons by the disk of Jupiter, and he noticed that during the year his predictions ran behind the actual eclipses he would see at the telescope.

Since the satellites had well-determined orbits with fixed orbit times around Jupiter, he correctly deduced that the delay between his prediction and the actual eclipse had to do with the difference in time it took for light to get to earth as the distance of Jupiter from the earth changed during the year. The amount of time was of the order of an hour or so throughout the year. Knowing the distance to Jupiter from the astronomical ephimerides, and the time delay, he estimated the speed of light to be 299,792,458 metres per second. Albert Einstein's theory of relativity is built on the premise that the speed of light in a vacuum is always the same.