This is only about 0. In the s, French physicist Jean Foucault measured the speed of light in a laboratory using a light source, a rapidly rotating mirror and a stationary mirror.
This method was based on a similar apparatus built by Armand-Hippolyte Fizeau. For the first time the speed of light could be measured on Earth, and the speed of light was measured to very great accuracy. In the s, interferometry was used to get the most accurate value for the speed of light that had been measured yet: , As a result, the numerical value of the speed of light c in meters per second is now fixed exactly by the definition of the meter.
It is always slower in other materials such as water or glass. Of course, Galileo points out that in fact nothing about the speed of light can be deduced from this observation, except that light moves faster than sound. He then goes on to suggest a possible way to measure the speed of light. The idea is to have two people far away from each other, with covered lanterns.
One uncovers his lantern, then the other immediately uncovers his on seeing the light from the first. This routine is to be practised with the two close together, so they will get used to the reaction times involved, then they are to do it two or three miles apart, or even further using telescopes, to see if the time interval is perceptibly lengthened.
From this one can certainly deduce that light travels at least ten times faster than sound. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. In September ,he correctly predicted that an eclipse on November 9 would be 10 minutes behind schedule.
This was indeed the case, to the surprise of his skeptical colleagues at the Royal Observatory in Paris. Two weeks later, he told them what was happening: as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io actually reflected sunlight, of course took time to reach the earth, and took the longest time when the earth was furthest away. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter.
Of course, to find the speed of light it was also necessary to know the distance from the earth to the sun. This very slight shift could be used to find the distance of Mars from earth, and hence the distance to the sun, since all relative distances in the solar system had been established by observation and geometrical analysis.
According to Crowe Modern Theories of the Universe , Dover, , page 30 , they concluded that the distance to the sun was between 40 and 90 million miles. The next substantial improvement in measuring the speed of light took place in , in England.
An astronomer James Bradley, sailing on the Thames with some friends, noticed that the little pennant on top of the mast changed position each time the boat put about, even though the wind was steady. Another possible analogy is to imagine the starlight as a steady downpour of rain on a windless day, and to think of yourself as walking around a circular path at a steady pace.
The apparent direction of the incoming rain will not be vertically downwards—more will hit your front than your back. In fact, if the rain is falling at, say, 15 mph, and you are walking at 3 mph, to you as observer the rain will be coming down at a slant so that it has a vertical speed of 15 mph, and a horizontal speed towards you of 3 mph.
Whether it is slanting down from the north or east or whatever at any given time depends on where you are on the circular path at that moment. Bradley reasoned that the apparent direction of incoming starlight must vary in just this way, but the angular change would be a lot less dramatic.
That meant that the angular variation in apparent incoming direction of starlight was about the magnitude of the small angle in a right-angled triangle with one side 10, times longer than the other, about one two-hundredth of a degree.
It would be reassuring to measure the speed of a beam of light between two points on the ground, rather than making somewhat indirect deductions based on apparent slight variations in the positions of stars.
We can see, though, that if the two lanterns are ten miles apart, the time lag is of order one-ten thousandth of a second, and it is difficult to see how to arrange that. He correctly surmised that this is due to the varying length of time it takes for light to travel from Jupiter to Earth as the distance between these two planets varies. In James Bradley made another estimate by observing stellar aberration, being the apparent displacement of stars due to the motion of the Earth around the Sun.
He observed a star in Draco and found that its apparent position changed throughout the year. All stellar positions are affected equally in this way. This distinguishes stellar aberration from parallax, which is greater for nearby stars than it is for distant stars. To understand aberration, a useful analogy is to imagine the effect of your motion on the angle at which rain falls past you, as you run through it.
If you stand still in the rain when there is no wind, it falls vertically on your head. If you run through the rain, it comes at you at an angle, and hits you on the front. The first measurement of c that didn't make use of the heavens was by Armand Fizeau in He used a beam of light reflected from a mirror 8 km away. The beam was aimed at the teeth of a rapidly spinning wheel.
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