1919] on Giant Suns 418 



measure the size of a threepenny-bit two miles off ; and we remember 

 his delight when his chairman on one occasion asked, " Who but a 

 Scotsman would care about a threepenny-bit two miles away ? " 



But you will see that this star is the nearest and therefore easiest 

 of all ; while some even of these brightest stars gave no result even 

 to a patient Scotsman. Those that are measurable show that they 

 are so far off that light takes years to come to us from them — from 

 some four years ; from others hundreds of years ; from those with 

 no measurable result, thousands of years at least. 



Thus we began to obtain a little knowledge of the distance of the 

 stars. The method used for these measurements was the usual 

 method of Parallax, which we may illustrate by two searchlights 

 trained on the same Zeppelin. Knowing their distance apart and the 

 angles at which they are sending their beams of light, those working 

 tlie apparatus can draw the triangle to scale and thus tell the height 

 of the Zeppelin. 



Xow replace the two searchlights by two telescopes- — one on one 

 side of the earth's orljit round the sun, and the other on the other 

 side ; they cannot be there simultaneously, but the star will wait six 

 months for us to move round or even longer. The angle at the 

 Zeppelin becomes however woefully small as we suppose it to mount 

 to the stars. It is twice the angle which seems to separate earth and 

 sun as seen from the Zeppelin, and it is this angle which is re- 

 presented by the diameter of " a threepenny-bit two miles away." 

 From the distance of the nearest star the sun and earth might appear 

 as a close double star, of which there are many examples in the 

 heavens, though our little earth would probably be too faint to be 

 seen, even from the nearest star-spectator. 



There would be no such difficulty in seeing the sun, but since his 

 diameter is only y^o^^ P^i'l^ of the distance between earth and sun, 

 which has itself shrunk to almost imperceptible dimensions, it is easy 

 to realise that the disc of the sun would have disappeared completely, 

 as does any disc of the stars to us, even with our largest telescopes. 



Since we have imagined ourselves to mount far upwards to a 

 Centauri, whence the sun and earth would represent a close double 

 star, let us retain the conception a moment longer in order to note a 

 useful fact. Watching long enough we should see the pair moving 

 across the heavens^ while at the same time the earth would revolve 

 round its mighty companion. The sun would proceed therefore 

 very much more steadily than the earth. The sun's path is nearly 

 straight, while the earth takes a wavy path, or more correctly a 

 corkscrew path. If the masses of the two bodies were more nearly 

 equal, the two paths would be more nearly alike, and both wavy. 



By observing such movements (for we can observe the movements 

 of stars) we infer whether one component of a double star is more 

 massive than the other or whether they are nearly equal ; and it is 

 found that there is never any very great disparity in mass between 



