Determination of the Speed of Light: How Did Ole Roemer Discover Speed of Light?

Around 400 years ago, scientists believed that the speed of light was infinite. But a Danish astronomer disproved this 340 years ago.

Around 400 years ago, scientists believed that the speed of light was infinite.  But a Danish astronomer disproved this 340 years ago.

To celebrate this discovery, Google came up with a doodle on December 7 showing astronomer Ole Rømer working at his laboratory.

In 1676, Rømer became the first person to measure the speed of light, the American Museum of Natural History says on its website.

Rømer came upon this discovery while working at the Paris Observatory. At the time, he was doing research on the orbit of Io, one of Jupiter’s four satellites. He wanted to determine a more accurate value for the satellite’s orbital period by timing its eclipses.

The Io completes its orbit around Jupiter in 1.769 Earth days. Jupiter eclipsed Io once every orbit, as seen from the earth.

The Danish astronomer conducted the observation for several years. He saw that the time interval between the eclipses decreased steadily as the Earth move closer to Jupiter. As soon as, the Earth moves away from Jupiter, the eclipses’ time intervals increased steadily.

The data Rømer gathered made him realize that the speed of light is finite, contrary to the popular belief at the time. This finding meant that one could measure the speed of light.

Rømer’s Findings Help Determine Speed of Light

“In a brilliant insight, he realized that the time difference must be due to the finite speed of light,” according to the AMNH.

This showed that the light from the Jupiter system travels farther to reach the Earth when the two planets are on the opposite sides of the Sun. The reverse holds true when the two are closer together.

Rømer then estimated that light crossed the diameter of the Earth’s orbit in 22 minutes. The speed of light can be measured by dividing the diameter of the Earth’s orbit by the time difference, he concluded.

Many scientists did not readily accept this finding. They continued to believe that the speed of light is infinite.

Later, Dutch scientist Christiaan Huygens estimated the speed of light at 131,000 miles per second. However, the correct value is 186,000 miles per second.

Huygens’s estimate was off because it was based on Rømer’s estimate for the maximum time delay of 22 minutes. The correct value is 16.7.  The error was also due to an imprecise knowledge of the Earth’s orbital diameter.

But the AMNH  recognized the importance of Rømer’s findings. The Danish astronomer’s findings ‘provided the first quantitative estimate for the speed of light.’ “And it was in the right ballpark,” it adds.

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