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Ole Rømer

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Ole Rømer was a Danish astronomer who demonstrated that the speed of light had a finite velocity by observing the orbital periods of Jupiter's moon Io. Working at the Royal Observatory in Paris, he calculated in 1676 that light took approximately 22 minutes to cross the diameter of Earth's orbit. Beyond this achievement, he served as the Royal Mathematician in Copenhagen, where he invented the transit telescope and modernized the city's systems for weights and measures.

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What did Ole Rømer discover about the speed of light?

Ole Rømer was the first astronomer to demonstrate that the speed of light is finite rather than instantaneous. Working at the Royal Observatory in Paris, he presented his findings in 1676 based on observations of Jupiter's moon Io.

How did Ole Rømer measure the speed of light using Jupiter's moon Io?

Rømer noticed that Io's eclipses by Jupiter appeared delayed when Earth was moving away from Jupiter and arrived earlier when Earth was approaching. He attributed the timing differences to the varying distance light had to travel across Earth's orbit.

How close was Rømer's calculated speed of light to the modern value?

Rømer calculated that light took approximately 22 minutes to cross the diameter of Earth's orbit, whereas the modern value is closer to about 16 minutes and 40 seconds. His number was not exact, but his core conclusion that light travels at a finite speed was correct and groundbreaking.

Where did Ole Rømer work as an astronomer?

Rømer worked primarily at the Royal Observatory in Paris under the French Academy of Sciences alongside astronomers like Giovanni Domenico Cassini. He later returned to Denmark, where he became a professor of astronomy at the University of Copenhagen and designed observatory instruments.