The Night Sky: Jupiter Eclipse = Speed of Light

Thanks to persistent observations of Jupiter’s moon Io, we know the speed of light.


Attached is a picture of Jupiter taken at the Mary Aloysia Hardey Observatory on January 11, 2013. It shows Jupiter’s Red Spot near the limb at the 4 o’clock position on Jupiter’s disk. Multi-colored cloud bands of this gas giant can be seen swirling and whirling much like eddies in a river.  Jupiter’s moons Ganymede and Io are labeled. Ganymede is casting its black shadow onto Jupiter’s cloud tops in an other-worldly eclipse.

Although a view like this is beautiful and interesting on its own, Jupiter’s moons have played an important role in the history of astronomy. For millennia no one could determine if the speed of light was finite or infinite. From Aristotle to Galileo, great minds were either simply wrong, or could not prove they were correct. Experiments with lamps on distant hill tops or using lunar eclipses were not potent enough to measure the incredibly fast speed of light.

It was not until 1676, when Danish astronomer Ole Rømer proved that the speed of light was finite. He noticed that Jupiter’s moon Io took longer to appear where predicted as the Earth moved farther from Jupiter. Rømer correctly concluded that this delay was caused by the time it took light to travel the longer distance as the Earth moved away from Jupiter in its orbit.

Rømer had the advantage of using the diameter of the Earth’s orbit (186 million miles) as a speed of light measuring tool.  It takes light about 16 minutes to travel the diameter of the Earth’s orbit. That length of time could be measured accurately enough with the clocks of Rømer’s time period, and that is why Rømer succeeded in proving that light has a finite speed. Rømer’s predicted light speed wasn’t completely accurate due to limitations in orbital data during his time period. Others later refined the speed of light to the 186,000 miles a second we take for granted today; but Ole Rømer’s discovery was a great leap forward for science.

Starry Nights!

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