SQi IKCTURE XLty. 



have occasioned a sensible annual parallax, or a change of their relative 

 situation, according to the earth's place in its orbit round the sun; for if this 

 orbit, viewed from any of the stars, subtended an angle even of a single 

 second, the place of that star might be observed to vary a second at diiferent 

 times of the year. Dr. Hooke supposed at one time that he had discovered 

 such a parallax, but later observations have not confirmed those of Dr. 

 Hooke. The stars have, however, a small aberration, in consetjuence of the 

 progressive motion of the earth in its orbit, combined with the limited 

 velocity of light; and the standard of comparison being the earth's axis, its 

 nutation must also in some degree affect the apparent places of the stars. It 

 Avas in endeavouring to ascertain the annual parallax, that Dr. Bradley 



discovered both the aberration of light and the nutation of the earth's 



axis. 



The revolution of the earth, in its orbit round the sun, produces the ap- 

 parent motion of the sun among the stars, by which he describes his annual 

 path in the ecliptic, with an apparent angular velocity equal to the angular 

 velocity of the earth, which varies considerably at various times. It required 

 some investigation of the magnitudes and distances of the heavenly bodies, 

 to be convinced that the sun and stars had not in reality the motion which 

 a superficial inspection of the heavens would naturally lead a spectator to 

 attribute to them; but it is at present perfectly unnecessary to enter into 

 arguments to prove that the true cause of these apparent motions is the real 

 motion of the earth. The effect of the earth's annual revolution is the 

 change of place of the sun among the fixed stars: it is obvious that -the 

 sun will always appear, when viewed from the earth, in a place diametrically 

 opposite to that in which the earth would appear, if seen from the sun: con- 

 sequently, since the earth and sun remain in the same plane, the apparent 

 path of the sun will mark the same circle among the stars as the earth would 

 appear to describe, if viewed from the sun, that is, the ecliptic. If the light 

 of the stars were much stronger, or that of the sun much weaker, we might 

 see him pass by the stars in each part of the ecliptic, as we do the moon ; but 

 we are now obliged to observe what stars are in turn diametrically opposite 

 to the sun, or at certain distances from him, and thus we obtain a correct 

 knowledge of his path. 



