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SIK FRANK W. DYSON, M.A., F.R.S., ON 



sun. As a consequence of this the light of a star does not seem 

 to come to us always from exactly the same direction. This is 

 easily illustrated by the familiar example of how an umbrella is 

 held in a shower of rain. Suppose the rain to be coming straight 

 down but if you are walking north you point your umbrella a 

 little to the north, if east a little to the east, because your move- 

 ment in combination with that of the raindrops makes them 

 appear to come in a direction slightly diverted from their true 

 one. Thus a star is not seen in its true direction, but in one 

 slightly diverted towards that in which the earth is moving. 

 This was not the discovery for which Bradley was working, but 

 it gave a method of measuring the velocity of light, and more 

 than that, it vindicated the Copernican theory in another 

 manner, for it showed that the earth was moving round the 

 sun. 



Another great astronomer, William Herschel, made a system- 

 atic search for the evidence of the nearness of some of the stars. 

 With his great telescope he searched for stars which seemed to 

 be near together ; he then used the following argument : here 

 are two stars which appear to be close togethc}*, but one may be 

 much further away than the other; it is in fact very likely that 

 the brighter star will be nearer to us than the fainter star. If 

 I have both these stars in my telescope at the same time, and 

 measure the angular distance between them, I may hope to find 

 that the nearer star changes its distance slightly from the further 

 star, due to the fact that the movement of the earth round the 

 sun sometimes brings the near star more into line with the 

 further star than at others. And witli my big telescope the 

 matter would not be desperate, even if the nearer star were as 

 much as 200,000 times the distance of the sun from us. The 

 argument was perfectly sound, but he did iiot find any stars so 

 near. He was rewarded by finding in the sky double stars, 

 which circulated round one another. For example, Castor 

 consists of two close stars which revolve round one anotlier, 

 though it takes hundreds of years for them to complete a 

 revolution. Many other attempts were made by astronomers, 

 and, curiously enough, success was achieved almost simultane- 

 ously about the year 1833 by Henderson at the Cape Observa- 

 tory, Struve at the National Observatory of Eussia at Pulkowa, 

 and Bessel at the Observatory at Konigsberg. Henderson found 

 the distance of the star a Centauri, one of the brightest stars 

 in the southern constellations. This star is 250,000 times as 

 far away as the sun. Struve found the distance of the bright 

 star Vega. This star is about 600,000 times as far as the sun. 



