154 



THE EARTH. 



expect to be conscious of this motion from anything which can be observed on 

 our own bodies or those which surround us on the surface of the earth : we 

 must look for it elsewhere. 



But it will be contended that the apparent motion of the sun, even upon the 

 argument just stated, may equally be explained by the motion of the earth 

 round the sun, or the motion of the sun round the earth ; and that therefore this 

 appearance can still prove nothing positively on this question. We have, how- 

 ever, other proofs, of a very decisive character. 



Newton showed that it was a general law of nature, and part, in fact, of the 

 principle of gravitation, that any two globes placed at a distance from each 

 other, if they are in the first instance quiescent, and free, must move with an 

 accelerated motion to their common centre of gravity, where they will meet 

 and coalesce ; but if they be projected in a direction not passing through this 

 centre of gravity, they will both of them revolve in orbits around that point 

 periodically. And in fact the same will be the case with any number of 

 globes whatsoever, and consequently would be applicable to the solar system 

 itself. 



Now, the centre of gravity of the solar system, owing to the immense pre- 

 dominance of the mass of the sun over all the rest of the bodies composing it 

 put together, is situated within the sun, and near its centre. All the bodies 

 of the system, and the earth among them, must therefore, according to this law, 

 revolve periodically round that point. 



But as the principle of gravitation itself may by some be considered as based 

 upon some previous admission of the motion of the planets, it may be desirable 

 to obtain a still more direct and positive manifestation of the annual motion of 

 the earth. Fortunately, the discovery which has been developed by the labors 

 of astronomical observers have put us in possession of a decisive test, which 

 has been considered as setting at rest for ever the question of the earth's an- 

 nual motion. If the earth were moved round the sun as it certainly must be 

 if the sun is not moved round it an effect would be produced upon the apparent 

 position of the fixed stars, owing to the combination of the motion of light with 

 the motion of the globe. Light is propagated from the stars in straight lines 

 with a velocity of about two hundred thousand miles per second. The earth, 

 if it moves at all, moves with a velocity of about twenty miles per second ; 

 and with this velocity, the eye of the observer upon the earth strikes the light 

 in the direction of the earth s motion, while the light itself comes in another 

 direction. The direction in which the observer will see the star will be de- 

 termined by the combined effect of the velocity of light and the velocity of the 

 earth, inasmuch as the impact of the light upon the eye will be the result of 

 these two motions ; thus, if the earth moved with a velocity equal to that of 

 light, the star would be seen forty-five degrees in advance of its real position. 

 If the earth moved with a less velocity, it would be seen less in advance of 

 its true position in proportion to the relative velocity of the earth and light. 



Now, the velocity of the earth being incomparably smaller than that of light, 

 the star ought to be seen in advance of its true position to an extent which is 

 proportionate to this small ratio, and the deviation of the star or planet's true 

 position should also be in the direction of the earth's motion. This effecl, 

 moreover, should be found to be produced upon all stars and planets visible in 

 the firmament ; modified, however, in a certain complicated manner, according 

 to their position with respect to the orbit of the earth. 



The observations of Bradley and subsequent astronomers detected these 

 effects ; and as they are everywhere produced upon the countless myriads of 

 objects that glitter upon the firmament, and everywhere produced in the manner 

 and degree exactly in which they ought to be produced by the earth's an- 



