Galileo, Aristotle and their Relevance to Home Lighting

Aristotle was a clever guy.So too, for that matter, was Descartes. Nevertheless both seemed to be of the view, now known to be mistaken, that light travelled instantaneously. Galileo, though, was not convinced.In his work Two New Scientists he had Simplicio reciting the Aristotelian position thus:

Everyday experience shows that the propagation of light is instantaneous; for when we see a piece of artillery fired at great distance, the flash reaches our eyes without lapse of time; but the sound reaches the ear only after a noticeable interval.

Galileo’s argument was that nothing meaningful could be deduced about the speed of light from this observation other than that light moves more quickly than sound and, perhaps, too quickly for the human eye to discern.He went on to conduct his own very rudimentary experiment to try to establish whether a delay could be detected at distances of up to a few miles, without success.

It was Romer in 1676 who really cracked it. He conducted a study of Io, one of Jupiter’s moons, and successfully tested a theory that the timing of each eclipse of Io by Jupiter could be accurately predicted by factoring in the distance that existed between Earth and Jupiter at the time when it occurred. In other words, with both planets moving in their own separate orbits the distance between them would vary, and the times at which the eclipse would be recorded would vary accordingly. Thus we could be confident that there was indeed a time delay between the emission of light when at a significant distance and the point at which it is seen.

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Lighting in our solar system and in our living rooms

In due course our understanding of this phenomenon improved further through the works of Bradley, Fizeau, Foucault and Michelson. Today we know that light travel at 299,792,458 metres per second (a precise figure, as the measure of a metre is actually defined by this particular constant), or about 186,000 miles per second. To give some idea of the speeds involved, radio waves travelling at the speed of light from astronauts on the moon reached us with a delay of approximately one second, whilst light from the sun reaches us within about eight and a half minutes.

Considering this timescale we are unlikely to notice any significant delay from the flicking of a switch to the appearance of our home lighting in our hallways and living rooms. Whether it be pendant lighting, chandeliers, table lamps or any other, they all do the job pretty much as soon as they are asked.

Thus we are free to measure the value of our own home lighting by its aesthetic qualities, by its performance and, maybe today, by its energy efficiency. Without having to worry too much about any delay in it illuminating our home, Galileo’s concerns notwithstanding

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