00:00 On this day in space.
00:03 On November 21st, 1676, the Danish astronomer Ole Røymer discovered the speed of light.
00:09 Before Røymer figured it out, scientists thought that light travels instantaneously, or infinitely fast.
00:15 Røymer disproved this almost by accident when he was studying Jupiter's moon Io.
00:20 He was trying to figure out how long it takes Io to orbit Jupiter in hopes of using it as a cosmic clock.
00:25 He watched Io disappear behind Jupiter and reappear on the other side.
00:29 He did this over and over every 42 hours for years.
00:33 To his surprise, the timing of the eclipses was not consistent.
00:37 When Earth was closest to Jupiter, the eclipses happened 11 minutes early.
00:41 Likewise, when the two planets were farthest away, the eclipses were 11 minutes behind schedule.
00:46 Røymer figured out the pattern and made an accurate prediction for Io's eclipse on November 9th, 1676.
00:52 Then on November 21st, he took his findings to the Royal Academy of Sciences and explained that a finite speed of light must be responsible.
00:59 And that's what happened on this day in space.
01:03 [ music ]
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