Thursday Throughput: Io Edition

Ole Romer’s illustration of measuring the speed of light.

[ThTh1] Everyone knows that light is the fastest thing in the Universe. Nothing can move faster than light and the laws of physics get very strange when you get anywhere close to it. In fact, light moves so fast that it took us a few millennia of scientific endeavor to find that it moved at all. Aristotle, among others, thought light moved instantaneously. So how was the speed of light first measured?

Galileo had one of the more famous ideas, although it’s not clear if he ever tried to execute it. Imagine you wanted to measure the speed of sound. Here’s one way you could do it. Have two people stand a mile apart. Have one fire a starter pistol and start a stopwatch. When the other person hears the pistol, they fire theirs. When the first participant hears that second report, he stops the watch. You then divide two miles by the time it took for the sound to travel back and forth to allow the watch to be stopped (about nine seconds). Do this and you’ll have the speed of sound to reasonable accuracy. I’ve done a rough estimate of this at Penn State football games from “We Are!” cheers and…I wasn’t far off.

Galileo proposed doing this for light using covered lanterns. One person would open his lantern. When the other saw the lantern, he would open his. The problem is that light moves too damn fast for this. It takes 5 millionths of a second for light to travel a mile. So, in the end, you’d only be measuring reaction time.

In the 17th century, however, Ole Romer figured out a practical way to measure the speed of light. It depended on Jupiter’s moon Io, which passes behind the planet and into its shadow on a regular basis. The time at which it passes into Jupiter’s shadow is easily calculated from its known orbit. However, the time at which we see it pass into that shadow depends on how far away Jupiter is. Olmer timed Io’s eclipses and found that they were occurring later than expected when Jupiter was at its furthest from us — about 10-15 minutes later, depending on the time of year. This gave him the speed of light relative to Earth’s distance from the Sun and later a number of 220,000 kilometers per second, not that far off from the modern 300,000.

The Era of Classical Astronomy — from Galileo through Newton — was full of these kinds of elegant experiments to measure the shape and size of the solar system, the distance to the nearest stars and the speed of light.

[ThTh2] For the record, this was not me. Although I appreciate everyone who checked in just to be sure.

[ThT3] More evidence that plastic recycling is an ecological fiasco. At this point, you’re better off putting your plastic in a nice landfill than recycling it.

[ThTh4] So how big is the Milky Way? Depends on what we’re talking about. The stars of the Milky Way occupy a disk about 100,000 light years across. But a new paper measured the dark matter halo in which the Milky Way is embedded and found it to be 1.9 million years across, extending halfway to the Andromeda galaxy.

[ThTh5] More evidence emerges of a star being torn apart by a black hole.

[ThTh6] Was the coronavirus made in a lab? Almost certainly not.

[ThTh7] The bad news is that the oceans are taking a beating from climate change, overfishing and pollution. The good news is they can improve rapidly and some recovery is already taking place.

[ThTh8] The antarctic once hosted rain forests.