214 SCIENCE AND METHOD. 



might imagine that we have the means of calculating 

 the absolute velocity of the Earth. (I shall explain the 

 meaning of this word " absolute " later.) But it is not 

 so at all. We certainly know the apparent position of 

 the star we are observing, but we do not know its true 

 position. We know the velocity of light only in terms 

 of magnitude and not of direction. 



If, therefore, the Earth's velocity were rectilineal and 

 uniform, we should never have suspected the pheno- 

 menon of aberration. But it is variable : it is composed 

 of two parts — the velocity of the Solar System, which 

 is, as far as we know, rectilineal and uniform ; and the 

 velocity of the Earth in relation to the Sun, which is 

 variable. If the velocity of the Solar System — that is 

 to say the constant part — alone existed, the observed 

 direction would be invariable. The position we should 

 thus observe is called the mean apparent position of 

 the star. 



Now if we take into account at once both parts of 

 the Earth's velocity, we shall get the actual apparent 

 position, which describes a small ellipse about the 

 mean apparent position, and it is this ellipse that is 

 observed. 



Neglecting very small quantities, we shall see that 

 the dimensions of this ellipse depend only upon the 

 relation between the Earth's velocity in relation to the 

 Sun and the velocity of light, so that the relative 

 velocity of the Earth in relation to the Sun is alone 

 in question. 



We must pause, however. This result is not exact, 

 but only approximate. Let us push the approxima- 

 tion a step further. The dimensions of the ellipse will 

 then depend upon the absolute velocity of the Earth. 



