On this day in space! Nov. 21, 1676: Astronomer (accidentally) discovers speed of light

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On Nov. 21, 1676, the Danish astronomer Ole Rømer discovered the speed of light.

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 of using it as a cosmic clock. He watched Io disappear behind Jupiter and reappear on the other side. He did this over and over every 42 hours for years.

A portrait of Danish astronomer Ole Rømer (Image credit: Wikimedia Commons)

Rømer figured out the pattern and made an accurate prediction for Io's eclipse on Nov. 9, 1676. Then on Nov. 21, he took his findings to the Royal Academy of Sciences and explained that a finite speed of light must be responsible.

On This Day in Space: See our full 365-day video archive!

Hanneke Weitering
Hanneke Weitering
Contributing expert

Hanneke Weitering is a multimedia journalist in the Pacific Northwest reporting on the future of aviation at FutureFlight.aero and Aviation International News and was previously the Editor for Spaceflight and Astronomy news here at Space.com. As an editor with over 10 years of experience in science journalism she has previously written for Scholastic Classroom Magazines, MedPage Today and The Joint Institute for Computational Sciences at Oak Ridge National Laboratory. After studying physics at the University of Tennessee in her hometown of Knoxville, she earned her graduate degree in Science, Health and Environmental Reporting (SHERP) from New York University. Hanneke joined the Space.com team in 2016 as a staff writer and producer, covering topics including spaceflight and astronomy. She currently lives in Seattle, home of the Space Needle, with her cat and two snakes. In her spare time, Hanneke enjoys exploring the Rocky Mountains, basking in nature and looking for dark skies to gaze at the cosmos. 

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