178 SIR DAVID GILL, ON THE SIDEREAL UNIVERSE. 



that point will be the apex towards which our sun is moving 

 amongst the stars. This apex is not far from the well-known 

 star Vega or a Lyrae. 



We have now considered the direction of motion of the sun, 

 and must pass on to consider the velocity of that motion. 



I think you are aware that the spectroscope can be applied to 

 determining the velocity of the motion of stars in the line of 

 sight. In the limited time at my disposal I cannot stop to 

 explain the theory of the spectroscopic method of determining 

 stellar velocities in the line of sight. I must ask you to take the 

 statements from me on faith. A spectrum of the star a Centauri 

 was here shown, taken at two epochs six months apart, that is to 

 say when in one instance the earth in its orbit round the sun 

 is moving towards a Centauri, and again six months afterwards 

 when it is moving away from it. The difference in the position or 

 displacement of the dark lines in the star's spectrum relative to 

 the corresponding bright lines of the spectrum of iron was 

 well shown on the screen. This displacement of the iron lines 

 in the star's spectrum relative to the corresponding lines in the 

 terrestrial spectrum of iron is accounted for by the fact that you 

 are encountering more waves of light in a second of time when 

 you are approaching the star than when you are receding from 

 it. From the measured displacements the velocity of the earth 

 in its motion round the sun, and therefore the distance of the 

 earth from the sun can be accurately calculated. When the 

 effect of the earth's motion round the sun is thus known, it can 

 be eliminated, and thus the velocity of any star's motioi], 

 referred to our sun, can be determined by measuring the dis- 

 placement of its spectral lines relative to corresponding lines 

 of terrestrial spectra. 



I have told you that the sun is moving towards the star Vega, 

 and if Vega were at rest it would be easy to ascertain the sun's 

 velocity in space. But Vega is itself moving, and we do not 

 know the direction or velocity of its motion. How, then, are 

 we to find out the velocity of the sun's motion ? We can only 

 do this by referring to a great number of stars, and imagine that 

 in any particular region of the sky the stars are moving 

 accidentally in all sorts of directions, but in no particular 

 direction in the mean. 



To make this more clear, let us return to our analogy of the 

 swarm of bees. If the queen bee is at rest and all the bees are 

 flying about her, the whole swarm keeping its general globular 

 form, then we might reasonably assume their mean motion in 

 space to be zero with reference to the queen-bee ; otherwise, the 



