Tag Archives: science

When did man learn the speed of light?

The history of science has all sorts of interesting stories. When did scientists determine that the speed of light was finite?

What brought this to my attention was talking to a person, an apparently well educated American teacher, who didn’t know whether the speed of light was infinite or finite.

In ancient times, some famous  people argued that light was emitted from the eyes. (Actually it’s more complicated than that.) Euclid, the father of Geometry, and Ptolemy, the famous astronomer,  both accepted this viewpoint. Consequently they argued that the speed of light was infinite, for when we open our eyes we can see the most-distant star instantly.

This viewpoint dominated for more than sixteen hundred years. Remember Galileo? He dropped the two cannon balls from the top of the leaning tower of pizza to prove all objects fell at the same speed?  Well he tried to determine the speed of light and concluded, “If it is not infinite it must be extraordinarily rapid.”

Think about this.  Gutenberg had created the printing press. Luther had started the reformation. Columbus had discovered the New World.  This is the period the American colonies were created.

Then along came a Danish astronomer working at the Royal Observatory in Paris in 1676. Ole Rømer was observing one of the moons of Jupiter, Io.  This moon orbits Jupiter in 42.5 hours and it winks on/off as it passes into Jupiter’s shadow.

Ole Rømer noticed a variation in the time of this winking on/off throughout the year. The periods of Io appeared to be shorter when the Earth was approaching Jupiter than when receding from it.

See this BBC youtube on this

He estimated the speed of light at 220,000 kilometers per second. (Actually he didn’t us kilometers  because they hadn’t been invented yet.)  Think of that speed.  Something moving at that speed would go around the earth five and a half times in a second.  No wonder some people didn’t accept his figure. In reality he was about 25% under the actual speed.

So, to prove that the speed of light was finite,  a scientist needed a reliable clock, a telescope, and  Jupiter’s moon Io.

How important is the speed of light? The Special Theory of Relativity rests on it. All physics and astronomy rests on it.



To see some of my short stories go to www.edwardmcdermott.net