On November 21, 1676, Danish astronomer Ole Remer discovered the speed of light. See how in our “This Day in Space” series. On November 21, 1676, Danish astronomer Ole Remer discovered the speed of light. Before Roemer realized this, scientists believed that light travels all at once, or at infinite speed.

Roemer disproved this almost by accident while studying Jupiter’s moon Io. He was trying to find out how long it took Io to orbit Jupiter, hoping to use it as a cosmic clock. He watched Io disappear behind Jupiter and reappear on the other side. It did this every 42 hours for years. The moon Io appears to be swirling above Jupiter’s clouds in this distant image from the Cassini spacecraft.

He was surprised by the inconsistency of the timing of the eclipses. Eclipses occurred 11 minutes ago when Earth was closest to Jupiter. Likewise, the eclipse was 11 minutes behind schedule when the two planets were furthest away. Roemer found the model and gave an accurate prediction for the Io eclipse on November 9, 1676. He then presented his findings to the Royal Academy of Sciences on 21 November, explaining that this must be due to the limited speed of light.