[00:02] to one of the most important discoveries in the history of physics. You see, Io goes dark when it falls into Jupiter's shadow, which it does every cycle cuz Jupiter is just so big. As it emerges, it becomes bright again. Watching it
[00:14] through a telescope night after night, you can mark the exact time that it pops back into view. And if you go out with a telescope and you do this a whole bunch, what you'll notice is that these happen in 42-hour increments, like clockwork.
[00:26] different times of the of the year, Io schedule. So again to draw a picture, Earth goes on the sun. Jupiter also goes on the sun. Yeah. So when you imagine the orbit
[00:40] sun. Yeah. So when you imagine the orbit of Io, um it was 20 minutes ahead of 20 the same side as as Jupiter. Then when the Earth was on the opposite side of Jupiter, it took 20 minutes longer. There was a delay in the orbit. So he
[00:55] measured this up after some very precise calculations and Huygens realized that the reason for this is because light was taking 20 minutes to traverse this extra distance. So two astronomical
[01:09] units is traversed in about 20 minutes. That's so clever. Yeah. Yeah. Yeah. actually know the true astronomical unit. So their estimates for the speed of light frankly don't look that impressive if you see them. But at that
[01:22] time, it was not even obvious that light had a speed. For most experiments you just looks instantaneous. But at really slow. And this very clever way to measure its speed by observing Io,
[01:35] imprecise though it was, laid the groundwork for more precise experiments down on Earth once people had at least a rough sense of how fast it really size.