SIR DAVID GILL_, ON THE SIDEREAL UNIVERSE. 



181 



with recent astronomical measures, and I told them that one- 

 hundredtli of a second of arc was the smallest stellar parallax 

 that an astronomer could measure. The angle to be measured is 

 equivalent to about the one-hundredth part of a threepenny bit 

 viewed a mile off. At the dinner which followed, the gentleman 

 who proposed my health said that there was no doubt about 

 the nationality of their President, because no one but a Scotch- 

 man would trouble his head about the hundredth part of a 

 threepenny bit a mile off! 



From the distance of a Centauri, light, which travels about 

 186,000 miles a second, would occupy 4 J years in reaching the 

 sun or our earth. Astronomers speak of the distance that light 

 travels in a year as a light-year; thus a Centauri would be 

 distant 4^ light-years. If one could travel to a Centauri at a 

 penny a hundred miles it would cost one and a-half times our 

 national debt for a single ticket ! Sirius is about twice that 

 distance from us, and so we go on till we get to the limit when 

 ordinary observations stop. But we have been able to measure 

 up to distances of 300 or 400 light years. 



The following table gives you, according to Professor Kapteyn, 

 the outcome of the combination of all known data bearing on 

 the distances of the stars and their distribution in space. Within 

 a sphere whose radius is 550 light-years (a distance corres- 

 ponding with that of an average ninth magnitude star) there 

 exist — 



1 star giving from 100,000* to 10,000* times the light of the sun. 

 46 stars „ 10,000* „ 1,000* 



1,300 „ „ 1,000* ,, 100* 



22,000 „ „ 100* „ 10- 



140,000 „ „ 10- „ 1* 



430,000 „ „ 1* „ -1 



650,000 „ „ -1 „ -01 



This shows that our sun is not a very important star ; and, 

 indeed, if it was viewed from the distance of the average first 

 magnitude star it would only appear as a star of the fifth 

 magnitude. 



Our earth, therefore, is a very insignificant planet revolving 

 round a very insignificant sun. 



I now pass to the consideration of the constituents of the 

 heavens. 



You all know that irregularly shaped band of light forming a 

 great circle in the sky called the Milky Way. I cannot, of 

 course, show you pictures of the whole of it, but here are 



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