Welcome to "On This Day ... in Space!" where we peer back in our archives to find historic moments in spaceflight and astronomy. So enjoy a blast from the past with Space.com's Hanneke Weitering to look back at what happened on this day in space! On Nov. 21, 1676, the Danish astronomer Ole Rømer discovered the speed of light . Before Rømer figured it out, scientists thought that light travels instantaneously, or infinitely fast. Rømer disproved this almost by accident when he was studying Jupiter's moon Io. He was trying to figure out how long it takes Io to orbit Jupiter in hopes...