Oct. 26, 1883.1 



KNOWLEDGE 



257 



THE SUN'S DISTANCE.* 



By Professor E. S. Ball, LL.D., 



astbonomer-koyal foe ireland. 

 (Continued from page 199.) 



ONE of the most celebrated methods of measuring the 

 distance of the sun is derived from a coraliination 

 of experiments on the velocity of light with astronomical 

 measurements. This is a method of very great refinement 

 and beauty, and although it does not satisfy all the neces- 

 sary conditions, it is impossible in this lecture to avoid a 

 brief reference to an investigation so remarkable. 



The principle of this method is easily enunciated. Though 

 the velocity of light is stupendous, yet it has been possible 

 to measure that velocity by experiment. The best deter- 

 minations indicate that a ray of light would flash over a 

 distance equal to seven times the circumference of the 

 earth in a single second of time (300,400 kilometres). A 

 ray of light would travel from Southport to London in 

 about the thousandth part of a second. The dimensions 

 of the solar system are, however, so considerable that even 

 a sunbeam requires an appreciable interval of time to span 

 the abyss which separates the earth from the sun. Eight 

 minutes is approximately the duration of the journey. In 

 fact, if the sun were to be suddenly blotted out, it would 

 still be seen shining brilliantly for eight minutes after it 

 had really disappeared. Viewed as a means of measuring 

 the sun's distance, it would be necessary to determine this 

 interval accurately to half a second. 



Does astronomy hold out any prospect of our being able 

 to measure this interval with such nicety ? This question 

 is intimately connected with two of the most brilliant dis- 

 coveries in the annals of astronomy. Although they are 

 well known to every one who has paid any attention to 

 this science, yet I must refer to them briefly to examine 

 how far they admit of the needful precision. 



Every one who has had the opportunity of using an 

 astronomical telescope is familiar with the exquisite system 

 of four moons which grace the great planet Jupiter. These 

 little bodies led 200 years ago to the discovery of the 

 velocity of light. In their frequent revolutions the little 

 satellites plunge occasionally into the vast and dense 

 shadow thrown by the mighty planet. The sun's light 

 which had previously rendered the little satellite a 

 -glittering point of light is then cut oil", the satellite be- 

 comes invisible, and we say that it is eclipsed. This is a 

 most pleasing phenomenon to witness, and as the satellites 

 revolve so rapidly, the eclipses occur with great frequency. 

 From the discussion of previous observations of the satel- 

 lites we have become aware of the nature of their move- 

 ments, so that the advent of eclipses can be predicted, and 

 «ven the time of their occurrence. But the time at which 

 the eclipse takes place is not identical with the moment 

 at which wc see the eclipse. There is an old story of the 

 French King who came to the observatory to see an eclipse ; 

 unfortunately his Majesty was late, the eclipse had passed, 

 and when this was explained to him, he wanted to know 

 if it could not be done over again. The only way in 

 which an eclipse could be done over again would be first to 

 view the eclipse from Jupiter himself, and then, with the 

 quickness of thought, to be transferred to the earth, from 

 whence the very same eclipse would be seen a second time. 

 As long as the little satellite is shining it radiates a stream 

 of light across the vast space between Jupiter and the 

 earth. When the eclipse has commenced, the little orb is 

 no longer luminous, but there is, nevertheless, a long 



* A discoiu-se delivered at the second geneial uioetiug of the 

 British Association at Southport. 



slender stream of light on its way, and until all this has 

 poured into our telescopes we stUl see the little satellite 

 shining as before. If we could calculate the moment when 

 the eclipse really took place, and if we could observe the 

 moment at which the eclipse is seen, the diflPerence between 

 the two gives the time which the light occupies on the 

 journey. At both extremities of the process there are 

 characteristic sources of uncertainty. The occurrence of 

 the eclipse is not an instantaneous phenomenon. The little 

 satellite is large enough to require an appreciable time in 

 crossing the boundary which defines the shadow, so that 

 the observation of an eclipse is not sufliciently precise to 

 form the basis of an important and accurate measurement. 

 Still greater difficulties accompany the attempt to define 

 the true moment of the occurrence of the eclipse as it 

 would be seen by an observer in the vicinity of the satellite. 

 For this we would require a far more perfect theory of the 

 movements of Jupiter's satellites than is at present 

 attainable. This method of finding the sun's distance 

 holds out no prospect of a result accurate t3 the one 

 thousandth part of its amount, and therefore we may 

 discard it. 



But there is yet another method by which the experi- 

 mental determination of the velocity of light can be so 

 combined with purely astronomical measurements as to 

 yield the sun's distance as a result. Time will not permit 

 that I should enter into this subject, which is identified 

 with one of the most glorious discoveries of astronomy — 

 namely, the aberration of light. But here I would only 

 remark that this method does not fulfil the condition of 

 geometrical necessity. It has not even the somewhat 

 inferior attribute of dynamical necessity. The physical 

 conceptions introduced may to some degree be open to 

 question. Is it certain that the velocity of light in air 

 must be the same as the velocity of light through space? 

 Is it certain that the motion of the whole solar system can 

 be suitably allowed for and eliminated ? The time has not 

 yet arrived when this latter question can be fully 



answered. 



(To be continued.) 



The Sunday Lecture Society. — The fourteenth annual 

 meeting of this society was held on Friday, October ."), at 

 43, Doughty-street, Holborn, under the presidency of Mr. 

 W. H. Domville, the hon. treasurer. Mr. M. E. Marsden, 

 the hon. secretary, read the report for the past year, from 

 which it appeared that in the session 1882-83, 21 lectures 

 had been delivered by lecturers eminent in science, history, 

 literature, and art, which were attended in the aggregate 

 by 10,587 persons, as against 9,80(5 attending 24 lectures 

 in the preceding session. After payment of all expenses, 

 there remained a balance in hand of £ 1 4 odd. There was 

 a slight increase in the number of members. The com- 

 mittee noted with much satisfaction the continued 

 success of kindred associations at Birmingham and 

 Glasgow, and of efforts to establish similar societies 

 in Edinburgh, Newcastle-on-Tyne, and other large cities 

 and towns. The committee expressed great regret at 

 the loss by death of Mr. William Spottiswoode, the late 

 President of the lloyal Society, and a Vice-President of 

 the Sunday Lecture Society, as well as one of its earliest 

 members. He always took a warm interest in the work of 

 the society. On the motion of the chairman, who said the 

 society was linaucially in a better position than it had ever 

 been before, the report was unanimously adopted. Votes 

 of thanks to the treasurer and secretary, the lecturers, 

 stewards at the lectures, and the auditors closed the pro- 

 ceedings. 



