A planet that played a big role in measuring the speed of light stands just above the Moon this evening.

Brilliant Jupiter is the largest planet in the solar system. And it has a giant family of moons, including four that are about the size of our moon or bigger. The smallest and nearest is Io. It’s closer to Jupiter than our moon is to Earth. It orbits Jupiter once every 42 and a half hours. And that’s the key to the measurement of lightspeed.

In the 1670s, Danish astronomer Olé Roemer was trying to refine the measurements of Io’s orbit. Io passes “behind” Jupiter on each orbit, so it vanishes in Jupiter’s long shadow. The timing reveals the details of Io’s orbit.

After a few years of measurements, though, Roemer noticed something odd. When Earth was moving toward Jupiter, Io vanished a few minutes earlier than expected. And when Earth was moving away from Jupiter, it vanished a few minutes later.

Roemer decided the change was a result of the changing distance between Earth and Jupiter. When the distance was greater, it took longer for light to make the trip. And that meant the speed of light was limited — something that scientists weren’t sure about before.

Roemer calculated that speed at about 500 million miles per hour. That’s about 25 percent off modern calculations. Still, it was the first reasonable estimate of the speed of light — made with the help of Jupiter and one of its many moons.

Script by Damond Benningfield

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