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Today’s Google Doodle celebrates the first astronomer to measure the speed of light

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The tale of Rømer and the Io eclipse


Today’s Google Doodle pays homage to a discovery that has had a profound effect on astronomy and essentially all of our observations of the Universe. The animation depicts Ole Rømer, a Danish astronomer who became the first person to successfully measure the speed of light. Rømer’s discovery was published 340 years ago on this day and has since revolutionized how we study cosmic distance and how light travels.

Prior to Rømer’s measurements, astronomers thought that light speed was infinite, but a few were starting to think otherwise. One of those contrarians was Galileo, who even devised his own method for figuring this out. He proposed the idea of setting up two people with lanterns about a mile apart from each other. The observers could then manually cover and uncover the lanterns, to see if there was any lag in the light’s travel time. Of course, now that we know just how fast light is (you know, 186,282 miles per second) there’s absolutely no way that would have been successful.

The reason Rømer was able to figure it out was because his experiment was much bigger in scope. In 1676, he had been trying to measure the eclipses of Jupiter’s moon Io, which occur once every 1.769 Earth days. That’s when he noticed something odd. The timing between each eclipse became progressively shorter as Earth got closer to Jupiter on its orbit, and the timing became longer as Earth moved farther away. Rømer knew that the position of Earth shouldn’t have any physical effect on Io’s eclipses, so the phenomenon had to be explained by something else.

That’s when he realized he had just measured light speed. Light from Jupiter has a longer distance to travel when Earth is farthest away from the gas giant, explaining the longer intervals between eclipses. In fact, Rømer found that eclipses occurred 11 minutes earlier than predicted when Earth and Jupiter were at their closest, and 11 minutes later when the two planets were farthest away from each other.

Rømer and Dutch astronomer Christiaan Huygens tried calculating the speed of light exactly, though their measurements turned out to be a little off. But Rømer is still the first person to observe the finite speed of light, so we have him to thank for how we calculate the distance of stars and galaxies throughout the Universe today.