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DISCOVERY 



Eclipses of Jupiter's 



Satellites and Their Use 



for Determining the 



Velocity of Light 



By Sir Oliver Lodge 



There is some misunderstanding as to the way in 

 which the eclipses of Jupiter's satellites enable the 

 velocity of light to be determined. It is often thought 

 that there is a delay in these eclipses when the earth 

 is a long way from Jupiter, as compared with the times 

 of their recurrence when the earth is comparatively 

 near. The distance between the two positions is the 

 whole diameter of the earth's orbit, that is to say, 

 186,000,000 miles, and undoubtedly light takes about 

 sixteen minutes to traverse that distance. But the 

 observation is not made in that wa}-, nor is it feasible 

 to make it in that way. 



The satellites revolve round Jupiter like the hands 

 of a clock, and what can be observed is the gaining 

 or the losing of that clock. This gain or loss is at a 

 maximum when the earth is moving full speed to or 

 from Jupiter, that is to say, when it is at a mean 

 distance. There is no gain or loss observable when the 

 earth is either at a maximum or minimum distance. 

 The clock would then be observed to keep right time, 

 or, in other words, the satellites would complete their 

 revolution, and be eclipsed, in their true period. 

 Distance would make no difference. Speed only is 

 effective. The apparent variations in the time of 

 revolution would be experienced when the earth was 

 moving either away from Jupiter or towards it. If it 

 is moving away, the time of revolution would appear 

 longer than usual. If it is moving towards, the time 

 of revolution would appear shortened. 



The Analogy of Sound 



It is in fact a case of what is called the " Doppler 

 Effect," which is most easily observed in the case of 

 sound. The rate of vibration of a sounding body can 

 be estimated by the pitch of the note it emits. If we 

 are moving towards the source— or what is the same 

 thing, if the source of sound is moving towards us— 

 the rate of vibration appears quicker than usual : 

 which is the same as saying that the sound is raised in 

 pitch, or appears shriller. If, on the other hand, there 



is a mutual recession between source and observer 



as when we are on a train travelling away from a rail- 

 way whistle— the pitch of the note is flattened ; that 

 is, the rate of vibration seems to us slower than it 

 really is. 



It is easy to observe this in the case of sound, because 

 the velocity of sound is but moderate. It travels with 

 about the speed of a revolver bullet, at the rate of a 

 mile in five seconds. So that if we were on a carriage 

 travelling away from the sound at this pace we should 

 not hear the sound at all, for it would never overtake us. 

 That rate of travel is impracticable by any artificial 

 means of locomotion. But we can easily travel a 

 mile a minute, which is one-twelfth of that rate, and 

 accordingly a note would be lowered by about one- 

 twelfth of its rate of vibration. So that if it were 

 sounding the C in the treble clef, making 512 complete 

 vibrations a second, it would sound to us, if we were 

 travelling away from it at sixty miles an hour, as if 

 it were making, not 512, but 470 vibrations a second, 

 or something like that — a drop of nearly a whole tone. 



It is not easy to observe the same thing in the case 

 of light, because the velocity of light is so enormous : 

 nevertheless it occurs. And if by a spectroscope we 

 were able to determine the position of a line in the 

 spectrum with sufficient accuracy — that is to say, if 

 we were able to determine precisely the rate of ethereal 

 vibration which was responsible for the light — we should 

 observe a slight shift in one direction when we were 

 moving away from it, and a corresponding shift in the 

 other direction when we were moving towards it. 



The only carriage which is sufficiently quick to cope 

 with the velocity of light is the earth in its orbit, which 

 is vastly quicker than even a bullet, since it travels 

 nineteen miles in every second. And though even 

 this is almost insignificant in comparison with the 

 velocity of light, it is not quite insignificant. It is 

 the ten-thousandth part of the velocity of light ; and 

 instruments of precision will show the effect, which is 

 ver}^ well known and constantly allowed for. Some 

 stars approach us much more rapidly than that, and 

 the speed of their motion is determined by this very 

 means. Sir William Huggins was the first to do it, 

 and at the time of his observation he found that the 

 star Sirius was receding from us at a considerable 

 pace, I think as much as forty miles a second, which is 

 proved by the displacement of its sodium line, so that 

 the sodium in Sirius appeared to be vibrating more 

 slowly, and emitting light of slightly more orange tinge, 

 than sodium on the earth or any other body fixed with 

 reference to the observer. 



Sirius and its Companion 



It is true that this recession of Sirius did not con- 

 tinue for many years. In the course of time recession 

 was turned into approach, proving that the star was 

 revolving round an unseen companion — a companion 

 whose existence was therefore predicted or detected 

 on this evidence alone, and was afterwards seen by 

 means of the large telescope and keen eye of Alvan 



